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• Mixing inheritance mapping strategies in NHibernate

  - by MylesRip

  I have a rather large inheritance hierarchy in which some of the subclasses add very little and others add quite a bit. I don't want to map the entire hierarchy using either "table per class hierarchy" or "table per subclass" due to the size and complexity of the hierarchy. Ideally I'd like to mix mapping strategies such that portions of the hierarchy where the subclasses add very little are combined into a common table a la "table per class hierarchy" and subclasses that add a lot are broken out into a separate table. Using this approach, I would expect to have 2 or 3 tables with very little wasted space instead of either 1 table with lots of fields that don't apply to most of the objects, or 20+ tables, several of which would have only a couple of columns. In the NHibernate Reference Documentation version 2.1.0, I found section 8.1.4 "Mixing table per class hierarchy with table per subclass". This approach switches strategies partway down the hierarchy by using: ... <subclass ...> <join ...> <property ...> ... </join> </subclass> ... This is great in theory. In practice, though, I found that the schema was too restrictive in what was allowed inside the "join" element for me to be able to accomplish what I needed. Here is the related part of the schema definition: <xs:element name="join"> <xs:complexType> <xs:sequence> <xs:element ref="subselect" minOccurs="0" /> <xs:element ref="comment" minOccurs="0" /> <xs:element ref="key" /> <xs:choice minOccurs="0" maxOccurs="unbounded"> <xs:element ref="property" /> <xs:element ref="many-to-one" /> <xs:element ref="component" /> <xs:element ref="dynamic-component" /> <xs:element ref="any" /> <xs:element ref="map" /> <xs:element ref="set" /> <xs:element ref="list" /> <xs:element ref="bag" /> <xs:element ref="idbag" /> <xs:element ref="array" /> <xs:element ref="primitive-array" /> </xs:choice> <xs:element ref="sql-insert" minOccurs="0" /> <xs:element ref="sql-update" minOccurs="0" /> <xs:element ref="sql-delete" minOccurs="0" /> </xs:sequence> <xs:attribute name="table" use="required" type="xs:string" /> <xs:attribute name="schema" type="xs:string" /> <xs:attribute name="catalog" type="xs:string" /> <xs:attribute name="subselect" type="xs:string" /> <xs:attribute name="fetch" default="join"> <xs:simpleType> <xs:restriction base="xs:string"> <xs:enumeration value="join" /> <xs:enumeration value="select" /> </xs:restriction> </xs:simpleType> </xs:attribute> <xs:attribute name="inverse" default="false" type="xs:boolean"> </xs:attribute> <xs:attribute name="optional" default="false" type="xs:boolean"> </xs:attribute> </xs:complexType> </xs:element> As you can see, this allows the use of "property" child elements or "component" child elements, but not both. It also doesn't allow for "subclass" child elements to continue the hierarchy below the point at which the strategy was changed. Is there a way to accomplish this?

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• Python: unable to inherit from a C extension.

  - by celil

  I am trying to add a few extra methods to a matrix type from the pysparse library. Apart from that I want the new class to behave exactly like the original, so I chose to implement the changes using inheritance. However, when I try from pysparse import spmatrix class ll_mat(spmatrix.ll_mat): pass this results in the following error TypeError: Error when calling the metaclass bases cannot create 'builtin_function_or_method' instances What is this causing this error? Is there a way to use delegation so that my new class behaves exactly the same way as the original?

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• Unique ID Defined by Most-Derived Class accessible through Base Class

  - by Narfanator

  Okay, so, the idea is that I have a map of "components", which inherit from componentBase, and are keyed on an ID unique to the most-derived*. Only, I can't think of a good way to get this to work. I tried it with the constructor, but that doesn't work (Maybe I did it wrong). The problem with any virtual, etc, inheritance tricks are that the user has to impliment them at the bottom, which can be forgotten and makes it less... clean. *Right phrase? If - is inheritance; foo is most-derived: foo-foo1-foo2-componentBase Here's some code showing the problem, and why CRTP can't cut it: (No, it's not legit code, but I'm trying to get my thoughts down) #include<map> class componentBase { public: virtual static char idFunction() = 0; }; template <class T> class component : public virtual componentBase { public: static char idFunction(){ return reinterpret_cast<char>(&idFunction); } }; class intermediateDerivations1 : public virtual component<intermediateDerivations1> { }; class intermediateDerivations2 : public virtual component<intermediateDerivations2> { }; class derived1 : public intermediateDerivations1 { }; class derived2 : public intermediateDerivations1 { }; //How the unique ID gets used (more or less) std::map<char, componentBase*> TheMap; template<class T> void addToMap(componentBase * c) { TheMap[T::idFunction()] = c; } template<class T> T * getFromMap() { return TheMap[T::idFunction()]; } int main() { //In each case, the key needs to be different. //For these, the CRTP should do it: getFromMap<intermediateDerivations1>(); getFromMap<intermediateDerivations2>(); //But not for these. getFromMap<derived1>(); getFromMap<derived2>(); return 0; } More or less, I need something that is always there, no matter what the user does, and has a sortable value that's unique to the most-derived class. Also, I realize this isn't the best-asked question, I'm actually having some unexpected difficultly wrapping my head around it in words, so ask questions if/when you need clarification.

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• Java Stack class inherit Vector Class

  - by SonOfOmer

  By extending class Vector, Java’s designers were able to create class Stack quickly. What are the negative aspects of this use of inheritance, particularly for class Stack? Thanks a lot.

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• Why can't I inherit static classes?

  - by User

  I have several classes that do not really need any state. From the organizational point of view, I would like to put them into hierarchy. But it seems I can't declare inheritance for static classes. Something like that: public static class Base { } public static class Inherited : Base { } will not work. Why have the designers of the language closed that possibility?

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• Problem with extending JPanel

  - by Halo

  I have an abstract entity: public abstract class Entity extends JPanel implements FocusListener And I have a TextEntity: public class TextEntity extends Entity Inside TextEntity's constructor I want to put a JTextArea that will cover the panel: textArea = new JTextArea(); textArea.setSize(getWidth(),getHeight()); add(textArea); But getWidth() and getHeight() returns 0. Is it a problem with the inheritance or the constructor?

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• Handling inheritance with overriding efficiently

  - by Fyodor Soikin

  I have the following two data structures. First, a list of properties applied to object triples: Object1 Object2 Object3 Property Value O1 O2 O3 P1 "abc" O1 O2 O3 P2 "xyz" O1 O3 O4 P1 "123" O2 O4 O5 P1 "098" Second, an inheritance tree: O1 O2 O4 O3 O5 Or viewed as a relation: Object Parent O2 O1 O4 O2 O3 O1 O5 O3 O1 null The semantics of this being that O2 inherits properties from O1; O4 - from O2 and O1; O3 - from O1; and O5 - from O3 and O1, in that order of precedence. NOTE 1: I have an efficient way to select all children or all parents of a given object. This is currently implemented with left and right indexes, but hierarchyid could also work. This does not seem important right now. NOTE 2: I have tiggers in place that make sure that the "Object" column always contains all possible objects, even when they do not really have to be there (i.e. have no parent or children defined). This makes it possible to use inner joins rather than severely less effiecient outer joins. The objective is: Given a pair of (Property, Value), return all object triples that have that property with that value either defined explicitly or inherited from a parent. NOTE 1: An object triple (X,Y,Z) is considered a "parent" of triple (A,B,C) when it is true that either X = A or X is a parent of A, and the same is true for (Y,B) and (Z,C). NOTE 2: A property defined on a closer parent "overrides" the same property defined on a more distant parent. NOTE 3: When (A,B,C) has two parents - (X1,Y1,Z1) and (X2,Y2,Z2), then (X1,Y1,Z1) is considered a "closer" parent when: (a) X2 is a parent of X1, or (b) X2 = X1 and Y2 is a parent of Y1, or (c) X2 = X1 and Y2 = Y1 and Z2 is a parent of Z1 In other words, the "closeness" in ancestry for triples is defined based on the first components of the triples first, then on the second components, then on the third components. This rule establishes an unambigous partial order for triples in terms of ancestry. For example, given the pair of (P1, "abc"), the result set of triples will be: O1, O2, O3 -- Defined explicitly O1, O2, O5 -- Because O5 inherits from O3 O1, O4, O3 -- Because O4 inherits from O2 O1, O4, O5 -- Because O4 inherits from O2 and O5 inherits from O3 O2, O2, O3 -- Because O2 inherits from O1 O2, O2, O5 -- Because O2 inherits from O1 and O5 inherits from O3 O2, O4, O3 -- Because O2 inherits from O1 and O4 inherits from O2 O3, O2, O3 -- Because O3 inherits from O1 O3, O2, O5 -- Because O3 inherits from O1 and O5 inherits from O3 O3, O4, O3 -- Because O3 inherits from O1 and O4 inherits from O2 O3, O4, O5 -- Because O3 inherits from O1 and O4 inherits from O2 and O5 inherits from O3 O4, O2, O3 -- Because O4 inherits from O1 O4, O2, O5 -- Because O4 inherits from O1 and O5 inherits from O3 O4, O4, O3 -- Because O4 inherits from O1 and O4 inherits from O2 O5, O2, O3 -- Because O5 inherits from O1 O5, O2, O5 -- Because O5 inherits from O1 and O5 inherits from O3 O5, O4, O3 -- Because O5 inherits from O1 and O4 inherits from O2 O5, O4, O5 -- Because O5 inherits from O1 and O4 inherits from O2 and O5 inherits from O3 Note that the triple (O2, O4, O5) is absent from this list. This is because property P1 is defined explicitly for the triple (O2, O4, O5) and this prevents that triple from inheriting that property from (O1, O2, O3). Also note that the triple (O4, O4, O5) is also absent. This is because that triple inherits its value of P1="098" from (O2, O4, O5), because it is a closer parent than (O1, O2, O3). The straightforward way to do it is the following. First, for every triple that a property is defined on, select all possible child triples: select Children1.Id as O1, Children2.Id as O2, Children3.Id as O3, tp.Property, tp.Value from TriplesAndProperties tp -- Select corresponding objects of the triple inner join Objects as Objects1 on Objects1.Id = tp.O1 inner join Objects as Objects2 on Objects2.Id = tp.O2 inner join Objects as Objects3 on Objects3.Id = tp.O3 -- Then add all possible children of all those objects inner join Objects as Children1 on Objects1.Id [isparentof] Children1.Id inner join Objects as Children2 on Objects2.Id [isparentof] Children2.Id inner join Objects as Children3 on Objects3.Id [isparentof] Children3.Id But this is not the whole story: if some triple inherits the same property from several parents, this query will yield conflicting results. Therefore, second step is to select just one of those conflicting results: select * from ( select Children1.Id as O1, Children2.Id as O2, Children3.Id as O3, tp.Property, tp.Value, row_number() over( partition by Children1.Id, Children2.Id, Children3.Id, tp.Property order by Objects1.[depthInTheTree] descending, Objects2.[depthInTheTree] descending, Objects3.[depthInTheTree] descending ) as InheritancePriority from ... (see above) ) where InheritancePriority = 1 The window function row_number() over( ... ) does the following: for every unique combination of objects triple and property, it sorts all values by the ancestral distance from the triple to the parents that the value is inherited from, and then I only select the very first of the resulting list of values. A similar effect can be achieved with a GROUP BY and ORDER BY statements, but I just find the window function semantically cleaner (the execution plans they yield are identical). The point is, I need to select the closest of contributing ancestors, and for that I need to group and then sort within the group. And finally, now I can simply filter the result set by Property and Value. This scheme works. Very reliably and predictably. It has proven to be very powerful for the business task it implements. The only trouble is, it is awfuly slow. One might point out the join of seven tables might be slowing things down, but that is actually not the bottleneck. According to the actual execution plan I'm getting from the SQL Management Studio (as well as SQL Profiler), the bottleneck is the sorting. The problem is, in order to satisfy my window function, the server has to sort by Children1.Id, Children2.Id, Children3.Id, tp.Property, Parents1.[depthInTheTree] descending, Parents2.[depthInTheTree] descending, Parents3.[depthInTheTree] descending, and there can be no indexes it can use, because the values come from a cross join of several tables. EDIT: Per Michael Buen's suggestion (thank you, Michael), I have posted the whole puzzle to sqlfiddle here. One can see in the execution plan that the Sort operation accounts for 32% of the whole query, and that is going to grow with the number of total rows, because all the other operations use indexes. Usually in such cases I would use an indexed view, but not in this case, because indexed views cannot contain self-joins, of which there are six. The only way that I can think of so far is to create six copies of the Objects table and then use them for the joins, thus enabling an indexed view. Did the time come that I shall be reduced to that kind of hacks? The despair sets in.

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• Implementing prototypes OR instantiating class objects

  - by intuited

  I'm wondering how to implement prototypal inheritance in Python. It seems like the way to do this would be to either use a metaclass to cause instantiations to actually be classes, rather than objects, or use some magical powers to transform an existing object into a class. The second method would seem to be more flexible, in that it could be applied to existing objects of varied types, while the first would likely be more convenient for typical use cases. Insights on the practicality of these two approaches, as well as alternative suggestions, are hereby requested.

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• How to access a superclass method from a nested class?

  - by m01

  I hope this code explains the problem: class Foo { void a() { / *stuff */ } } class Bar extends Foo { void a() { throw new Exception("This is not allowed for Bar"); } class Baz { void blah() { // how to access Foo.a from here? } } } I know that I may be doing something wrong, because inheritance perhaps shouldn't be used in such way. But it's the easiest way in my situation. And, beside that, I'm just curious. Is it possible?

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• Inheritance issue

  - by VenkateshGudipati

  hi Friends i am facing a issue in Inheritance i have a interface called Irewhizz interface irewhzz { void object save(object obj); void object getdata(object obj); } i write definition in different class like public user:irewhzz { public object save(object obj); { ....... } public object getdata(object obj); { ....... } } this is antoher class public client:irewhzz { public object save(object obj); { ....... } public object getdata(object obj); { ....... } } now i have different classes like public partial class RwUser { #region variables IRewhizzDataHelper irewhizz; IRewhizzRelationDataHelper irewhizzrelation; private string _firstName; private string _lastName; private string _middleName; private string _email; private string _website; private int _addressId; private string _city; private string _zipcode; private string _phone; private string _fax; //private string _location; private string _aboutMe; private string _username; private string _password; private string _securityQuestion; private string _securityQAnswer; private Guid _user_Id; private long _rwuserid; private byte[] _image; private bool _changepassword; private string _mobilephone; private int _role; #endregion //IRewhizz is the interface and its functions are implimented by UserDataHelper class //RwUser Class is inheriting the UserDataHelper Properties and functions. //Here UserDataHelper functions are called with Irewhizz Interface Object but not with the //UserDataHelper class Object It will resolves the unit testing conflict. #region Constructors public RwUser() : this(new UserDataHelper(), new RewhizzRelationalDataHelper()) { } public RwUser(IRewhizzDataHelper repositary, IRewhizzRelationDataHelper relationrepositary) { irewhizz = repositary; irewhizzrelation = relationrepositary; } #endregion #region Properties public int Role { get { return _role; } set { _role = value; } } public string MobilePhone { get { return _mobilephone; } set { _mobilephone = value; } } public bool ChangePassword { get { return _changepassword; } set { _changepassword = value; } } public byte[] Image { get { return _image; } set { _image = value; } } public string FirstName { get { return _firstName; } set { _firstName = value; } } public string LastName { get { return _lastName; } set { _lastName = value; } } public string MiddleName { get { return _middleName; } set { _middleName = value; } } public string Email { get { return _email; } set { _email = value; } } public string Website { get { return _website; } set { _website = value; } } public int AddressId { get { return _addressId; } set { _addressId = value; } } public string City { get { return _city; } set { _city = value; } } public string Zipcode { get { return _zipcode; } set { _zipcode = value; } } public string Phone { get { return _phone; } set { _phone = value; } } public string Fax { get { return _fax; } set { _fax = value; } } //public string Location //{ // get // { // return _location; // } // set // { // _location = value; // } //} public string AboutMe { get { return _aboutMe; } set { _aboutMe = value; } } public string username { get { return _username; } set { _username = value; } } public string password { get { return _password; } set { _password = value; } } public string SecurityQuestion { get { return _securityQuestion; } set { _securityQuestion = value; } } public string SecurityQAnswer { get { return _securityQAnswer; } set { _securityQAnswer = value; } } public Guid UserID { get { return _user_Id; } set { _user_Id = value; } } public long RwUserID { get { return _rwuserid; } set { _rwuserid = value; } } #endregion #region MemberFunctions // DataHelperDataContext db = new DataHelperDataContext(); // RewhizzDataHelper rwdh=new RewhizzDataHelper(); //It saves user information entered by user and returns the id of that user public object saveUserInfo(RwUser userObj) { userObj.UserID = irewhizzrelation.GetUserId(username); var res = irewhizz.saveData(userObj); return res; } //It returns the security questions for user registration } public class Agent : RwUser { IRewhizzDataHelper irewhizz; IRewhizzRelationDataHelper irewhizzrelation; private int _roleid; private int _speclisationid; private int[] _language; private string _brokaragecompany; private int _loctionType_lk; private string _rolename; private int[] _specialization; private string _agentID; private string _expDate; private string _regstates; private string _selLangs; private string _selSpels; private string _locations; public string Locations { get { return _locations; } set { _locations = value; } } public string SelectedLanguages { get { return _selLangs; } set { _selLangs = value; } } public string SelectedSpecialization { get { return _selSpels; } set { _selSpels = value; } } public string RegisteredStates { get { return _regstates; } set { _regstates = value; } } //private string _registeredStates; public string AgentID { get { return _agentID; } set { _agentID = value; } } public string ExpDate { get { return _expDate; } set { _expDate = value; } } private int[] _registeredStates; public SelectList RegisterStates { set; get; } public SelectList Languages { set; get; } public SelectList Specializations { set; get; } public int[] RegisterdStates { get { return _registeredStates; } set { _registeredStates = value; } } //public string RegisterdStates //{ // get // { // return _registeredStates; // } // set // { // _registeredStates = value; // } //} public int RoleId { get { return _roleid; } set { _roleid = value; } } public int SpeclisationId { get { return _speclisationid; } set { _speclisationid = value; } } public int[] Language { get { return _language; } set { _language = value; } } public int LocationTypeId { get { return _loctionType_lk; } set { _loctionType_lk = value; } } public string BrokarageCompany { get { return _brokaragecompany; } set { _brokaragecompany = value; } } public string Rolename { get { return _rolename; } set { _rolename = value; } } public int[] Specialization { get { return _specialization; } set { _specialization = value; } } public Agent() : this(new AgentDataHelper(), new RewhizzRelationalDataHelper()) { } public Agent(IRewhizzDataHelper repositary, IRewhizzRelationDataHelper relationrepositary) { irewhizz = repositary; irewhizzrelation = relationrepositary; } public void inviteclient() { //Code related to mailing } //DataHelperDataContext dataObj = new DataHelperDataContext(); //#region IRewhizzFactory Members //public List<object> getAgentInfo(string username) //{ // var res=dataObj.GetCompleteUserDetails(username); // return res.ToList(); // throw new NotImplementedException(); //} //public List<object> GetRegisterAgentData(string username) //{ // var res= dataObj.RegisteredUserdetails(username); // return res.ToList(); //} //public void saveAgentInfo(string username, string password, string firstname, string lastname, string middlename, string securityquestion, string securityQanswer) //{ // User userobj=new User(); // var result = dataObj.rw_Users_InsertUserInfo(firstname, middlename, lastname, dataObj.GetUserId(username), securityquestion, securityquestionanswer); // throw new NotImplementedException(); //} //#endregion public Agent updateData(Agent objectId) { objectId.UserID = irewhizzrelation.GetUserId(objectId.username); objectId = (Agent)irewhizz.updateData(objectId); return objectId; } public Agent GetAgentData(Agent agentodj) { agentodj.UserID = irewhizzrelation.GetUserId(agentodj.username); agentodj = (Agent)irewhizz.getData(agentodj); if (agentodj.RoleId != 0) agentodj.Rolename = (string)(string)irewhizzrelation.getValue(agentodj.RoleId); if (agentodj.RegisterdStates.Count() != 0) { List<SelectListItem> list = new List<SelectListItem>(); string regstates = ""; foreach (int i in agentodj.RegisterdStates) { SelectListItem listitem = new SelectListItem(); listitem.Value = i.ToString(); listitem.Text = (string)irewhizzrelation.getValue(i); list.Add(listitem); regstates += (string)irewhizzrelation.getValue(i) + ","; } SelectList selectlist = new SelectList(list, "Value", "Text"); agentodj.RegisterStates = selectlist; if(regstates!=null) agentodj.RegisteredStates = regstates.Remove(regstates.Length - 1); } if (agentodj.Language.Count() != 0) { List<SelectListItem> list = new List<SelectListItem>(); string selectedlang = ""; foreach (int i in agentodj.Language) { SelectListItem listitem = new SelectListItem(); listitem.Value = i.ToString(); listitem.Text = (string)irewhizzrelation.getValue(i); list.Add(listitem); selectedlang += (string)irewhizzrelation.getValue(i) + ","; } SelectList selectlist = new SelectList(list, "Value", "Text"); agentodj.Languages = selectlist; // agentodj.SelectedLanguages = selectedlang; } if (agentodj.Specialization.Count() != 0) { List<SelectListItem> list = new List<SelectListItem>(); string selectedspel = ""; foreach (int i in agentodj.Specialization) { SelectListItem listitem = new SelectListItem(); listitem.Value = i.ToString(); listitem.Text = (string)irewhizzrelation.getValue(i); list.Add(listitem); selectedspel += (string)irewhizzrelation.getValue(i) + ","; } SelectList selectlist = new SelectList(list, "Value", "Text"); agentodj.Specializations = selectlist; //agentodj.SelectedSpecialization = selectedspel; } return agentodj; } public void SaveImage(byte[] pic, String username) { irewhizzrelation.SaveImage(pic, username); } } now the issue is when ever i am calling agent class it is given error like null reference exception for rwuser class can any body give the solution thanks in advance

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• What are the alternatives to "overriding a method" when using composition instead of inheritance?

  - by Sebastien Diot

  If we should favor composition over inheritance, the data part of it is clear, at least for me. What I don't have a clear solution to is how overwriting methods, or simply implementing them if they are defined in a pure virtual form, should be implemented. An obvious way is to wrap the instance representing the base-class into the instance representing the sub-class. But the major downsides of this are that if you have say 10 methods, and you want to override a single one, you still have to delegate every other methods anyway. And if there were several layers of inheritance, you have now several layers of wrapping, which becomes less and less efficient. Also, this only solve the problem of the object "client"; when another object calls the top wrapper, things happen like in inheritance. But when a method of the deepest instance, the base class, calls it's own methods that have been wrapped and modified, the wrapping has no effect: the call is performed by it's own method, instead of by the highest wrapper. One extreme alternative that would solve those problems would be to have one instance per method. You only wrap methods that you want to overwrite, so there is no pointless delegation. But now you end up with an incredible amount of classes and object instance, which will have a negative effect on memory usage, and this will require a lot more coding too. So, are there alternatives (preferably alternatives that can be used in Java), that: Do not result in many levels of pointless delegation without any changes. Make sure that not only the client of an object, but also all the code of the object itself, is aware of which implementation of method should be called. Does not result in an explosion of classes and instances. Ideally puts the extra memory overhead that is required at the "class"/"particular composition" level (static if you will), rather than having every object pay the memory overhead of composition. My feeling tells me that the instance representing the base class should be at the "top" of the stack/layers so it receives calls directly, and can process them directly too if they are not overwritten. But I don't know how to do it that way.

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• Single Table Per Class Hierarchy with an abstract superclass using Hibernate Annotations

  - by Andy Hull

  I have a simple class hierarchy, similar to the following: @Entity @Table(name="animal") @Inheritance(strategy=InheritanceType.SINGLE_TABLE) @DiscriminatorColumn(name="animal_type", discriminatorType=DiscriminatorType.STRING) public abstract class Animal { } @Entity @DiscriminatorValue("cat") public class Cat extends Animal { } @Entity @DiscriminatorValue("dog") public class Dog extends Animal { } When I query "from Animal" I get this exception: "org.hibernate.InstantiationException: Cannot instantiate abstract class or interface: Animal" If I make Animal concrete, and add a dummy discriminator... such as @DiscriminatorValue("animal")... my query returns my cats and dogs as instances of Animals. I remember this being trivial with HBM based mappings but I think I'm missing something when using annotations. Can anyone help? Thanks!

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• Hibernate: Parent/Child relationship in a single-table

  - by Dee

  I hardly see any pointer on the following problem related to Hibernate. This pertains to implementing inheritance using a single database table with a parent-child relationship to itself. For example: CREATE TABLE Employee ( empId BIGINT NOT NULL AUTO_INCREMENT, empName VARCHAR(100) NOT NULL, managerId BIGINT, CONSTRAINT pk_employee PRIMARY KEY (empId) ) Here, the managerId column may be null, or may point to another row of the Employee table. Business rule requires the Employee to know about all his reportees and for him to know about his/her manager. The business rules also allow rows to have null managerId (the CEO of the organisation doesn't have a manager). How do we map this relationship in Hibernate, standard many-to-one relationship doesn't work here? Example code would be appreciated.

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• Hibernate: Parent/Child relationship in a single-table

  - by Dee

  I hardly see any pointer on the following problem related to Hibernate. This pertains to implementing inheritance using a single database table with a parent-child relationship to itself. For example: CREATE TABLE Employee ( empId BIGINT NOT NULL AUTO_INCREMENT, empName VARCHAR(100) NOT NULL, managerId BIGINT, CONSTRAINT pk_employee PRIMARY KEY (empId) ) Here, the managerId column may be null, or may point to another row of the Employee table. Business rule requires the Employee to know about all his reportees and for him to know about his/her manager. The business rules also allow rows to have null managerId (the CEO of the organisation doesn't have a manager). How do we map this relationship in Hibernate, standard many-to-one relationship doesn't work here? Example code would be appreciated.

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• How to treat an instance variable as an instance of another type in C#

  - by Ben Aston

  I have a simple inheritance heirarchy with MyType2 inheriting from MyType1. I have an instance of MyType1, arg, passed in as an argument to a method. If arg is an instance of MyType2, then I'd like to perform some logic, transforming the instance. My code looks something like the code below. Having to create a new local variable b feels inelegant - is there a way of achieving the same behavior without the additional local variable? public MyType1 MyMethod(MyType1 arg) { if(arg is MyType2) { MyType2 b = arg as MyType2; //use b (which modifies "arg" as "b" is a reference to it)... } return arg; }

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• How do you use the LINQ to SQL designer to generate accessor methods for subclasses?

  - by Pricey

  Above is the LINQ to SQL designer view for my data context. Below is the relevant code: public System.Data.Linq.Table<ActivityBase> ActivityBases { get { return this.GetTable<ActivityBase>(); } } ... [Table(Name="dbo.Activities")] [InheritanceMapping(Code="1", Type=typeof(ActivityBase), IsDefault=true)] [InheritanceMapping(Code="2", Type=typeof(Project))] [InheritanceMapping(Code="3", Type=typeof(ProjectActivity))] [InheritanceMapping(Code="5", Type=typeof(Task))] [InheritanceMapping(Code="4", Type=typeof(Activity))] public abstract partial class ActivityBase : INotifyPropertyChanging, INotifyPropertyChanged { ... Is there a way to generate accessor methods for the subclasses as shown in the inheritance mapping above (Project, Task, etc...) without doing it manually? I added them manually but then a change in the designer overwrites any manual changes. Am i doing this wrong? should I not be making accessors for the sub classes? filtering from ActivityBase seems worse to me. Thanks for any help on this.

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• Passing List (Of ChildClass) as parameter to method expecting List (Of ParentClass)?

  - by Nicholas

  Hi, I have implemented inheritance for two parent classes called Table and Field. Each parent class has several child classes. In the Table parent class, I created a method that expects parameter List(Of Field). I get an error when trying to pass in parameter List(Of ChildField) to the method that expects a parameter of List(Of Field). The error message is as below: Value of type 'System.Collections.Generic.List(Of com.hlb.icisbatch.data.ChildField)' cannot be converted to 'System.Collections.Generic.List(Of com.hlb.icisbatch.data.Field)' My question, is it possible to pass in list of child class as parameter? If it is not a list then it works fine. But somehow it is not possible with lists? Below is sample class structure: Table Field | | ChildTable ChildField I have a method in the parent class: Public Class Table Public Sub New() End Sub Public Overridable Sub setFields(ByVal list As List(Of Field) 'Do work here' End Sub End Class and method in child class: Public Class ChildTable Public Sub New(ByVal list As List(Of ChildField) setFields(ChildField) End Sub End Class

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• A general question about inheritence in the .NET framework

  - by Grant

  I have a general question about inheritance in the .NET framework, lets say you have 2 classes, the first is called Parent and the second is called Child. Child inherits from Parent. Parent wants to ensure that each instance of child executes a specific piece of code when it loads irrespective of whether the child has their own onLoad code explicitly specified. From my experience, if i handle onLoad in the parent and not in the child, the parents onLoad code will fire but if its handled in both classes only the child's code will fire. Is this correct? and if so how can i ensure the parents code will always fire for the child...

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• How do I find the "concrete class" of a django model baseclass

  - by Mr Shark

  I'm trying to find the actual class of a django-model object, when using model-inheritance. Some code to describe the problem: class Base(models.model): def basemethod(self): ... class Child_1(Base): pass class Child_2(Base): pass If I create various objects of the two Child classes and the create a queryset containing them all: Child_1().save() Child_2().save() (o1, o2) = Base.objects.all() I want to determine if the object is of type Child_1 or Child_2 in basemethod, I can get to the child object via o1.child_1 and o2.child_2 but that reconquers knowledge about the childclasses in the baseclass. I have come up with the following code: def concrete_instance(self): instance = None for subclass in self._meta.get_all_related_objects(): acc_name = subclass.get_accessor_name() try: instance = self.__getattribute__(acc_name) return instance except Exception, e: pass But it feels brittle and I'm not sure of what happens when if I inherit in more levels.

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• Mixing table per subclass and per hierarchy in hibernate

  - by Xelluloid

  In my database there are two three tables. The first one, table ABSTRACT, holds three columns id, type, someText This table contains all abstract information for the abstract class abstract. Now the two tables CONCRETEONE and CONCRETETWO contain all information for the concrete classes concreteOne and concreteTwo. Now I know I could use the table per subclass strategy from hibernate to create a mapping with inheritance. But as I have a column that marks the type of the concrete implementation could it be possible to create some mixed behaviour like a table per subclass strategy with an discriminator?

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• Objective-C Custom extend

  - by ryanjm.mp

  I have a couple classes that have nearly identical code. Only a string or two is different between them. What I would like to do is to make them from another class that defines those functions and then uses constants or something else to define those strings that are different. I'm not sure if "___" is inheritance or extending or what. That is what I need help with. For example: objectA.m: -(void)helloWorld { NSLog("Hello %@",child.name); } objectBob.m: #define name @"Bob" objectJoe.m #define name @"Joe" (I'm not sure if it's legal to define strings, but this gets the point across) It would be ideal if objectBob.m and objectJoe.m didn't have to even define the methods, just their relationship to objectA.m. Is there any way to do something like this? If all else fails I'll just make objectA.m: -(void)helloWorld:(NSString *name) { NSLog("Hello %@",name); } And have the other files call that function (and just #import objectA.m).

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• Override An Existing Property as a Child form of its return type

  - by Jason

  I apologize if that title is confusing. This question may be a result of lack of coffee and/or sleep, but my mind is not working correctly right now. Anyways, I have an inheritance tree like such (I know the architecture isn't ideal): BaseClass GeneralForm : Inherits BaseClass SpecificForm : Inherits GeneralForm And an object like so: MyItem MySpecificItem : Inherits MyItem I have Items As List (Of MyItem) as a property in BaseClass. I would like for SpecificForm to somehow override Items to return type List (Of MySpecificItem). I feel like this is easy to do, but again, my head is spinning and I can't think straight at the moment. Thank you so much in advance.

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• Map derived class as an independent one with FNH's Automap

  - by Anton Gogolev

  Hi! Basically, I have an ImageMetadata class and an Image class, which derives from ImageMetadata. Image adds one property: byte[] Content, which actually contains binary data. What I want to do is to map these two classes onto one table, but I absolutely do not need NHibernates' inheritance support to kick in. I want to tailor FNH Automap to produce something like: <class name="ImageMetadata" ...> <property name="Name" ... /> < ... /> <class name="Image" ...> <property name="Name" ... /> <property name="Content" ... /> < ... /> Is this at all possible?

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• C# Inherited member variables behaving undexpectedly

  - by Nilbert

  If I have a class like this: class A { public string fe = "A"; } And a class that inherits from it like so: class B : A { public string fe = "B"; } Visual C++ will tell me that B.fe hides A.fe so I should use the new keyword. So I change class B to look like: class B : A { public new string fe = "B"; } And then I have a function that takes an A (but, by virtue of inheritance, will also take a B) like this: class D { public static void blah(A anAObject) { Console.Writeline(A.fe); } } Even when I pass it an instance of a B object, which it will take without question, it will print "A"! Why is this, and how can I make it work how I want without setting the variable in the constructor?

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• Make Java parent class not part of the interface

  - by Bart van Heukelom

  (This is a hypothetical question for discussion, I have no actual problem). Say that I'm making an implementation of SortedSet by extending LinkedHashMap: class LinkedHashSortedMapThing extends LinkedHashMap implements SortedSet { ... } Now programmers who use this class may do LinkedHashMap x = new LinkedHashSortedMapThing(); But what if I consider the extending of LinkedHashMap an implementation detail, and do not want it to be a part of the class' contract? If people use the line above, I can no longer freely change this detail without worrying about breaking existing code. Is there any way to prevent this sort of thing, other than favouring composition over inheritance (which is not always possible due to private/protected members)?

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