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  • NHibernate Pitfalls: Lazy Scalar Properties Must Be Auto

    - by Ricardo Peres
    This is part of a series of posts about NHibernate Pitfalls. See the entire collection here. NHibernate supports lazy properties not just for associations (many to one, one to one, one to many, many to many) but also for scalar properties. This allows, for example, only loading a potentially large BLOB or CLOB from the database if and when it is necessary, that is, when the property is actually accessed. In order for this to work, other than having to be declared virtual, the property can’t have an explicitly declared backing field, it must be an auto property: 1: public virtual String MyLongTextProperty 2: { 3: get; 4: set; 5: } 6:  7: public virtual Byte [] MyLongPictureProperty 8: { 9: get; 10: set; 11: } All lazy scalar properties are retrieved at the same time, when one of them is accessed.

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  • Creating a Lazy Sequence of Directory Descendants in C#

    My dear friend Craig Andera posted an implementation of a function that descends into a directory in a "lazy" manner, i.e. you get the first descendant back right away and not after all descendants have been calculated. His implementation was in Clojure, a Lisp variant that runs on the Java VM: (import [java.io File])(defn dir-descendants [dir]  (let [children (.listFiles (File. dir))]    (lazy-cat      (map (memfn getPath) (filter (memfn isFile) children))...Did you know that DotNetSlackers also publishes .net articles written by top known .net Authors? We already have over 80 articles in several categories including Silverlight. Take a look: here.

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  • Stop lazy loading or skip loading a property in NHibernate? Proxy cannot be serialized through WCF

    - by HelloSam
    Consider I have a parent, child relationship class and mapping. I am using NHibernate to read the object from the database, and intended to use WCF to send the object across the wire. Goal For reading the parent object, I want to selectively, at different execution path, decide when I would want to load the child object. Because I don't want to read more than what I needed. Those partially loaded object must be able to sent through WCF. When I mean I don't load it, neither side will access such property. Problem When such partially loaded object is being sent through WCF, as those property is marked as [DataContract], it cannot be serialized as the property is lazy load proxy instead of real known type. What I want to archive, or solution that I can think of lazy=false or lazy=true doesn't work. Former will eagerly fetch all the relationships, latter will create a proxy. But I want nothing instead - a null would be the best. I don't need lazy load. I hope to get a null for those references that I don't want to fetch. A null, but not just a proxy. This will makes WCF happy, and waste less time to have a lazy-load proxy constructed. Like could I have a null proxy factory? -OR- Or making WCF ignoring those property that's a proxy instead of real. I tried the IDataContractSurrogate solution, but only parent is passed to GetObjectToSerialize, I never observe an proxy being passed through GetObjectToSerialize, leaving no chance to un-proxy it. Edit After reading the comments, more surfing on the Internet... It seems to me that DTO would shift major part of the computation to the server side. But for the project I am working on, 50% of time the client is "smarter" than the server and the server is more like a data store with validation and verification. Though I agree the server is not exactly dumb - I have to decide when to fetch the extra references already, and DTO will make this very explicit. Maybe I should just take the pain. I didn't know http://automapper.codeplex.com/ before, this motivates me a little more to take the pain. On the other hand, I found http://trentacular.com/2009/08/how-to-use-nhibernate-lazy-initializing-proxies-with-web-services-or-wcf/, which seems to be working with IDataContractSurrogate.GetObjectToSerialize.

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  • Lazy coding is fun

    - by Anthony Trudeau
    Every once in awhile I get the opportunity to write an application that is important enough to do, but not important enough to do the right way -- meaning standards, best practices, good architecture, et al.  I call it lazy coding.  The industry calls it RAD (rapid application development). I started on the conversion tool at the end of last week.  It will convert our legacy data to a completely new system which I'm working on piece by piece.  It will be used in the future, but only the new parts because it'll only be necessary to convert the individual pieces of the data once.  It was the perfect opportunity to just whip something together, but it was still functional unlike a prototype or proof of concept.  Although I would never write an application like this for a customer (internal or external) this methodology (if you can call it that) works great for something like this. I wouldn't be surprised if I get flamed for equating RAD to lazy coding or lacking standards, best practice, or good architecture.  Unfortunately, it fits in the current usage.  Although, it's possible to create a good, maintainable application using the RAD methodology, it's just too ripe for abuse and requires too much discipline for someone let alone a team to do right. Sometimes it's just fun to throw caution to the wind and start slamming code.

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  • what's the alternative to readonlycollection when using lazy="extra"?

    - by Kunjan
    I am trying to use lazy="extra" for the child collection of Trades I have on my Client object. The trades is an Iset<Trade Trades, exposed as ReadOnlyCollection<Trade because I do not want anyone to modify the collection directly. As a result, I have added AddTrade and RemoveTrade methods. Now I have a Client Search page where I need to show the Trade Count. and on the Client details page I have a tab where I need to show all the trades for the Client in paged gridview. What I want to achieve is, for the search when I say on the client object as client.Trades.Count, nHibernate should only fire a select count(*) query. Hence I am using lazy="extra". But because I am using a ReadOnlyCollection, nHibernate fires a count query & a separate query to load the child collection trades completely. Also, I cannot include the Trades in my initial search request as this would disturb the paging because a counterparty can have n trades which would result in n rows, when I am searching clients only. So the child collections have to be loaded lazily. The second problem is that on the client details page -- Trades grid view, I have enabled paging for performance reasons. But by nature nHibernate loads the entire collection of trades as the user goes from one page to another. Ideally I want to control this by getting only trades specific to the page the user is on. How can I achieve this? I came across this very good article. http://stackoverflow.com/questions/876976/implementing-ipagedlistt-on-my-models-using-nhibernate But I am not sure if this will work for me, as lazy=extra currently doesnt work as expected with the ReadOnlyCollection. So, if I went ahead and implemented the solution this way and further enhanced it by making the List/Set Immutable, will lazy=extra give me the same problem as with ReadOnlyCollections?

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  • Any way to define getters for lazy variables in Javascript arrays?

    - by LLer
    I'm trying to add elements to an array that are lazy-evaluated. This means that the value for them will not be calculated or known until they are accessed. This is like a previous question I asked but for objects. What I ended up doing for objects was Object.prototype.lazy = function(var_name, value_function) { this.__defineGetter__(var_name, function() { var saved_value = value_function(); this.__defineGetter__(var_name, function() { return saved_value; }); return saved_value; }); } lazy('exampleField', function() { // the code that returns the value I want }); But I haven't figured out a way to do it for real Arrays. Arrays don't have setters like that. You could push a function to an array, but you'd have to call it as a function for it to return the object you really want. What I'm doing right now is I created an object that I treat as an array. Object.prototype.lazy_push = function(value_function) { if(!this.length) this.length = 0; this.lazy(this.length++, value_function); } So what I want to know is, is there a way to do this while still doing it on an array and not a fake array?

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  • Lazy Processing of Streams

    - by Giorgio
    I have the following problem scenario: I have a text file and I have to read it and split it into lines. Some lines might need to be dropped (according to criteria that are not fixed). The lines that are not dropped must be parsed into some predefined records. Records that are not valid must be dropped. Duplicate records may exist and, in such a case, they are consecutive. If duplicate / multiple records exist, only one item should be kept. The remaining records should be grouped according to the value contained in one field; all records belonging to the same group appear one after another (e.g. AAAABBBBCCDEEEFF and so on). The records of each group should be numbered (1, 2, 3, 4, ...). For each group the numbering starts from 1. The records must then be saved somewhere / consumed in the same order as they were produced. I have to implement this in Java or C++. My first idea was to define functions / methods like: One method to get all the lines from the file. One method to filter out the unwanted lines. One method to parse the filtered lines into valid records. One method to remove duplicate records. One method to group records and number them. The problem is that the data I am going to read can be too big and might not fit into main memory: so I cannot just construct all these lists and apply my functions one after the other. On the other hand, I think I do not need to fit all the data in main memory at once because once a record has been consumed all its underlying data (basically the lines of text between the previous record and the current record, and the record itself) can be disposed of. With the little knowledge I have of Haskell I have immediately thought about some kind of lazy evaluation, in which instead of applying functions to lists that have been completely computed, I have different streams of data that are built on top of each other and, at each moment, only the needed portion of each stream is materialized in main memory. But I have to implement this in Java or C++. So my question is which design pattern or other technique can allow me to implement this lazy processing of streams in one of these languages.

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  • In Haskell, will calling length on a Lazy ByteString force the entire string into memory?

    - by me2
    I am reading a large data stream using lazy bytestrings, and want to know if at least X more bytes is available while parsing it. That is, I want to know if the bytestring is at least X bytes long. Will calling length on it result in the entire stream getting loaded, hence defeating the purpose of using the lazy bytestring? If yes, then the followup would be: How to tell if it has at least X bytes without loading the entire stream? EDIT: Originally I asked in the context of reading files but understand that there are better ways to determine filesize. Te ultimate solution I need however should not depend on the lazy bytestring source.

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  • Is it possible to match with decomposed sequences in F#?

    - by Ball
    I seem to remember an older version of F# allowing structural decomposition when matching sequences just like lists. Is there a way to use the list syntax while keeping the sequence lazy? I'm hoping to avoid a lot of calls to Seq.head and Seq.skip 1. I'm hoping for something like: let decomposable (xs:seq<'a>) = match xs with | h :: t -> true | _ -> false seq{ 1..100 } |> decomposable But this only handles lists and gives a type error when using sequences. When using List.of_seq, it seems to evaluate all the elements in the sequence, even if it is infinite.

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  • Lazy umount or Unmounting a busy disk in Linux

    - by deed02392
    I have read that it is possible to 'umount' a disk that is otherwise busy by using the 'lazy' option. The manpage has this to say about it: umount - unmount file systems -l Lazy unmount. Detach the filesystem from the filesystem hierarchy now, and cleanup all references to the filesystem as soon as it is not busy anymore. This option allows a "busy" filesystem to be unmounted. (Requires kernel 2.4.11 or later.) But what would be the point in that? I considered why we dismount partitions at all: To remove the hardware To perform operations on the filesystem that would be unsafe to do while mounted In either of these cases, all a 'lazy' unmount serves IMHO is to make it more difficult to determine if the disk really is dismounted and you can actually proceed with these actions. The only application for umount -l seems to be for inexperienced users to 'feel' like they've achieved something they haven't. Why would you use a lazy unmount?

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  • Lazy HTML attributes wrapping in Internet Explorer

    - by AGS777
    Having encountered this Internet Explorer (all versions) behavior several times previously, I eventually decided to share this most probably useless knowledge. Excuse my lengthy explanations because I am going to show the behavior along with a very simple case when one can come across it inadvertently. Let's say I want to implement some simple templating solution in JavaScript. I wrote an HTML template with an intention to bind data to it on the client side: Please note, that name of the “sys-template” class is just a coincidence. I do not use any ASP.NET AJAX code in this simple example. As you can see we need to replace placeholders (property name wrapped with curly braces) with actual data. Also, as you can see, many of the placeholders are situated within attribute values and it is where the danger lies. I am going to use <a /> element HTML as a template and replace each placeholder pattern with respective properties’ values with a little bit of jQuery like this: You can find complete code along with the contextFormat() method definition at the end of the post. Let’s assume that value for the name property (that we want to put in the title attribute) of the first data item is “first tooltip”. So it consists of two words. When the replacement occurred, title attribute should contain the “first tooltip” text which we are going to see as a tooltip for the <a /> element. But let’s run the sample code in Internet Explorer and check it out. What you’ll see is that only the first word of the supposed “title” attribute’s content is shown. So, were is the rest of my attribute and what happened? The answer is obvious once you see the result of jQuery(“.sys-template”).html() line for the given HTML markup. In IE you’ll get the following <A id={id} class={cssClass} title={name} href="{source}" myAttr="{attr}">Link to {source}</A> See any difference between this HTML and the one shown earlier? No? Then look carefully. While the original HTML of the <a /> element is well-formed and all the attributes are correctly quoted, when you take the same HTML back in Internet Explorer (it doesn’t matter whether you use html() method from jQuery library or IE’s innerHTML directly), you lose attributes’ quotes for some of the attributes. Then, after replacement, we’ll get following HTML for our first data item. I marked the attribute value in question with italic: <A id=1 class=first title=first tooltip href="first.html" myAttr="firstAttr">Link to first.html</A> Now you can easily imagine for yourself what happens when this HTML is inserted into the document and why we do not see the second (and any subsequent words if any) of our title attribute in the tooltip. There are still two important things to note. The first one (and it actually the reason why I named the post “lazy wrapping” is that if value of the HTML attribute does contains spaces in the original HTML, then it WILL be wrapped with quotation marks. For example, if I wrote following on my page (note the trailing space for the title attribute value) <a href="{source}" title="{name}  " id="{id}" myAttr="{attr}" class="{cssClass}">Link to {source}</a> then I would have my placeholder quoted correctly and the result of the replacement would render as expected: The second important thing to note is that there are exceptions for the lazy attributes wrapping rule in IE. As you can see href attribute value did not contain spaces exactly as all the other attributes with placeholders, but it was still returned correctly quoted Custom attribute myAttr is also quoted correctly when returned back from document, though its placeholder value does not contain spaces either. Now, on account of the highly unlikely probability that you found this information useful and need a solution to the problem the aforementioned behavior introduces for Internet Explorer browser, I can suggest a simple workaround – manually quote the mischievous attributes prior the placeholder pattern is replaced. Using the code of contextFormat() method shown below, you would need to add following line right before the return statement: result = result.replace(/=({([^}]+)})/g, '="$1"'); Below please find original sample code:

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  • See former key sequences in vim

    - by Vasiliy Sharapov
    Sometimes I share screen shots and clips of vim usage with others. It would be nice to expand on the part of the status bar highlighted in this picture: I would like some way to make previous key sequences visible as well, such as: y2w jj f[ p 2d - You can see the key sequences leading up to the current one. I'll elaborate on my wish list at the bottom. Is something like this is available as a plugin or vim script? The sheer number of scripts available on vim online makes this hard to find by keyword. Some features I would hope for (but seem improbable): Delimit key sequences with a non-keyboard character instead of space, and a different one for the current command, so y2w jj f[ p 2d might become y2w¦jj¦f[¦p » 2d Replace keys that have a letter alternative with the alternative, such as the right arrow key - ^[[C with the equivalent l. Edit: To clarify, the right arrow key is a valid key in vim, but has no character to represent it, the l key preforms the same function and could/should substitute it. Have previous keystrokes run all the way to the beginning of the line (instead of just one or two), and just have vim's command prompt overwrite it when necessary. Replace some keystrokes with a more elegant alternative, for example hhhhh with 5h or more impressively d2f) with d% (in the appropriate situation).

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  • What good books are out there on program execution models? [on hold]

    - by murungu
    Can anyone out there name a few books that address the topic of program execution models?? I want a book that can answer questions such as... What is the difference between interpreted and compiled languages and what are the performance consequences at runtime?? What is the difference between lazy evaluation, eager evaluation and short circuit evaluation?? Why would one choose to use one evaluation strategy over another?? How do you simulate lazy evaluation in a language that favours eager evaluation??

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  • OOP App Architecture: Which layer does a lazy loader sit in?

    - by JW
    I am planning the implemention an Inheritance Mapper pattern for an application component http://martinfowler.com/eaaCatalog/inheritanceMappers.html One feature it needs to have is for a domain object to reference a large list of aggreageted items (10,000 other domain objects) So I need some kind of lazy loading collection to be passed out of the aggregate root domain object to other domain objects. To keep my (php) model scripts organised i am storing them in two folders: MyComponent\ controllers\ models\ domain\ <- domain objects, DDD repository, DDD factory daccess\ <- PoEAA data mappers, SQL queries etc views\ But now I am racking my brains wondering where my lazy loading collection sits. Any suggestions / justifications for putting it in one place over another another? DDD = Domain Driven Design Patterns, Eric Evans - book PoEAA = Patterns of Application Architecture Patterns, Martin Fowler - book

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  • C# Unit Testing: How do I set a Lazy<T>.ValueCreated to false?

    - by michael paul
    Basically, I have a unit test that gets a singleton instance of a class. Some of my tests required me to mock this singleton, so when I do Foo.Instance I get a different type of instance. The problem is that my checks are passing individually, but failing overall because one test is interfering with another. I tried to do a TestCleanup where I set: Foo_Accessor._instance = null; but that didn't work. What I really need is Foo_Accessor._instance.IsValueCreated = false; (_instance is a Lazy). Any way to unset the Lazy object that I didn't think of?

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  • How are managed the sequences by JPA and Hibernate?

    - by romaintaz
    Hi all, I am using Hibernate in my project, and many of my entities use a sequence for their technical keys. For example: @Entity @Table(name = "T_MYENTITY") @SequenceGenerator(name = "S_MYENTITY", sequenceName = "S_MYENTITY") public class MyEntity { @Id @GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "S_MYENTITY") @Column(name = "MY_ENTITY_ID") private Long entityId; ... } I have two questions about the ID generated by Hibernate when a new object of this class is persisted: Why SequenceGenerator (from javax.persistence) has a default value of allocationSize set to 50 instead of 1? What are the interests of that? What is the default algorithm used by Hibernate to calculate the generated ID? It seems that Hibernate uses the value returned by the sequence hosted by my Oracle database, but then modify it before assigning it to my entity...

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  • Why does the entity framework need an ICollection for lazy loading?

    - by Akk
    I want to write a rich domain class such as public class Product { public IEnumerable<Photo> Photos {get; private set;} public void AddPhoto(){...} public void RemovePhoto(){...} } But the entity framework (V4 code first approach) requires an ICollection type for lazy loading! The above code no longer works as designed since clients can bypass the AddPhoto / RemovePhoto method and directly call the add method on ICollection. This is not good. public class Product { public ICollection<Photo> Photos {get; private set;} //Bad public void AddPhoto(){...} public void RemovePhoto(){...} } It's getting really frustrating trying to implement DDD with the EF4. Why did they choose the ICollection for lazy loading? How can i overcome this? Does NHibernate offer me a better DDD experience?

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  • Can lazy loading be considered an example of RAII?

    - by codemonkey
    I have been catching up on my c++ lately, after a couple years of exclusive Objective-C on iOS, and the topic that comes up most on 'new style' c++ is RAII To make sure I understand RAII concept correctly, would you consider Objective-C lazy loading property accessors a type of RAII? For example, check the following access method - (NSArray *)items { if(_items==nil) { _items=[[NSArray alloc] initWithCapacity:10]; } return _items } Would this be considered an example of RAII? If not, can you please explain where I'm mistaken?

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  • Mirroring git and mercurial repos the lazy way

    - by Greg Malcolm
    I maintain Python Koans on mirrored on both Github using git and Bitbucket using mercurial. I get pull requests from both repos but it turns out keeping the two repos in sync is pretty easy. Here is how it's done... Assuming I’m starting again on a clean laptop, first I clone both repos ~/git $ hg clone https://bitbucket.org/gregmalcolm/python_koans ~/git $ git clone [email protected]:gregmalcolm/python_koans.git python_koans2 The only thing that makes a folder a git or mercurial repository is the .hg folder in the root of python_koans and the .git folder in the root of python_koans2. So I just need to move the .git folder over into the python_koans folder I'm using for mercurial: ~/git $ rm -rf python_koans/.git ~/git $ mv python_koans2/.git python_koans ~/git $ ls -la python_koans total 48 drwxr-xr-x 11 greg staff 374 Mar 17 15:10 . drwxr-xr-x 62 greg staff 2108 Mar 17 14:58 .. drwxr-xr-x 12 greg staff 408 Mar 17 14:58 .git -rw-r--r-- 1 greg staff 34 Mar 17 14:54 .gitignore drwxr-xr-x 13 greg staff 442 Mar 17 14:54 .hg -rw-r--r-- 1 greg staff 48 Mar 17 14:54 .hgignore -rw-r--r-- 1 greg staff 365 Mar 17 14:54 Contributor Notes.txt -rw-r--r-- 1 greg staff 1082 Mar 17 14:54 MIT-LICENSE -rw-r--r-- 1 greg staff 5765 Mar 17 14:54 README.txt drwxr-xr-x 10 greg staff 340 Mar 17 14:54 python 2 drwxr-xr-x 10 greg staff 340 Mar 17 14:54 python 3 That’s about it! Now git and mercurial are tracking files in the same folder. Of course you will still need to set up your .gitignore to ignore mercurial’s dotfiles and .hgignore to ignore git’s dotfiles or there will be squabbling in the backseat. ~/git $ cd python_koans/ ~/git/python_koans $ cat .gitignore *.pyc *.swp .DS_Store answers .hg <-- Ignore mercurial ~/git/python_koans $ cat .hgignore syntax: glob *.pyc *.swp .DS_Store answers .git <-- Ignore git Because both my mirrors are both identical as far as tracked files are concerned I won’t yet see anything if I check statuses at this point: ~/git/python_koans $ git status # On branch master nothing to commit (working directory clean) ~/git/python_koans $ hg status ~/git/python_koans But how about if I accept a pull request from the bitbucket (mercuial) site? ~/git/python_koans $ hg status ~/git/python_koans $ git status # On branch master # Your branch is behind 'origin/master' by 1 commit, and can be fast-forwarded. # # Changed but not updated: # (use "git add <file>..." to update what will be committed) # (use "git checkout -- <file>..." to discard changes in working directory) # # modified: python 2/koans/about_decorating_with_classes.py # modified: python 2/koans/about_iteration.py # modified: python 2/koans/about_with_statements.py # modified: python 3/koans/about_decorating_with_classes.py # modified: python 3/koans/about_iteration.py # modified: python 3/koans/about_with_statements.py Mercurial doesn’t have any changes to track right now, but git has changes. Commit and push them up to github and balance is restored to the force: ~/git/python_koans $ git commit -am "Merge from bitbucket mirror: 'gpiancastelli - Fix for issue #21 and some other tweaks'" [master 79ca184] Merge from bitbucket mirror: 'gpiancastelli - Fix for issue #21 and some other tweaks' 6 files changed, 78 insertions(+), 63 deletions(-) ~/git/python_koans $ git push origin master Or just use hg-git? The github developers have actually published a plugin for automatic mirroring: http://hg-git.github.com I haven’t used it because at the time I tried it a couple of years ago I was having problems getting all the parts to play nice with each other. Probably works fine now though..

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  • Sorting by key > 10 integer sequences. with thrust

    - by smilingbuddha
    I want to perform a sort_by_key where I have a single key-sequence and multiple value sequences. One usually performs this with sort_by_key( key, key + N, make_zip_iterator( make_tuple(x1 , x2 , ...) ) ) However I want to perform a sort with 10 sequences each of length N. Thrust does not support tuples of size = 10. So is there a way around this ? Of course one can keep a separate copy of the key vector and perform sorts on bunches of 10 sequences. But I would like to do everything in a single call.

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  • Does Entity Framework 4 not support property automatic lazy loading for model-first entities?

    - by KallDrexx
    All references that I find for lazy loading say it's possible but they all mention POCOs and that's it. I am using EF4 with the model-first methodology. In my model diagram I have a Project table and a UserObject table, with a 1 to many relationship between them. However, in code, when I have a valid UserObject and I attempt to get the project performing: Project prj = userobj.Project. Unfortunately, this doesn't work as it claims that UserObject.Project is null. It seems like I have to explicitly load the Project object via calling UserObject.ProjectReference.Load() prior to calling .Project. Is there any way for this to occur automatically when I access the .Project property?

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