how to create an access d - Page 504

[EF + Oracle] Entities

- by JTorrecilla

Prologue Following with the Serie I started yesterday about Entity Framework with Oracle, Today I am going to start talking about Entities. What is an Entity? A Entity is an object of the EF model corresponding to a record in a DB table. For example, let’s see, in Image 1 we can see one Entity from our model, and in the second one we can see the mapping done with the DB. (Image 1) (Image 2) More in depth a Entity is a Class inherited from the abstract class “EntityObject”, contained by the “System.Data.Objects.DataClasses” namespace. At the same time, this class inherits from the following Class and interfaces: StructuralObject: It is an Abstract class that inherits from INotifyPropertyChanging and INotifyPropertyChanged interfaces, and it exposes the events that manage the Changes of the class, and the functions related to check the data types of the Properties from our Entity. IEntityWithKey: Interface which exposes the Key of the entity. IEntityWithChangeTracker: Interface which lets indicate the state of the entity (Detached, Modified, Added…) IEntityWithRelationships: Interface which indicates the relations about the entity. Which is the Content of a Entity? A Entity is composed by: Properties, Navigation Properties and Methods. What is a Property? A Entity Property is an object that represents a column from the mapped table from DB. It has a data type equivalent in .Net Framework to the DB Type. When we create the EF model, VS, internally, create the code for each Entity selected in the Tables step, such all methods that we will see in next steps. For each property, VS creates a structure similar to: · Private variable with the mapped Data type. · Function with a name like On{Property_Name}Changing({dataType} value): It manages the event which happens when we try to change the value. · Function with a name like On{Property_Name}Change: It manages the event raised when the property has changed successfully. · Property with Get and Set methods: The Set Method manages the private variable and do the following steps: Raise Changing event. Report the Entity is Changing. Set the prívate variable. For it, Use the SetValidValue function of the StructuralObject. There is a function for each datatype, and the functions takes 2 params: the value, and if the prop allow nulls. Invoke that the entity has been successfully changed. Invoke the Changed event of the Prop. ReportPropertyChanging and ReportPropertyChanged events, let, respectively, indicate that there is pending changes in the Entity, and the changes have success correctly. While the ReportPropertyChanged is raised, the Track State of the Entity will be changed. What is a Navigation Property? Navigation Properties are a kind of property of the type: EntityCollection<TEntity>, where TEntity is an Entity type from the model related with the current one, it is said, is a set of record from a related table in the DB. The EntityCollection class inherits from: · RelatedEnd: There is an abstract class that give the functions needed to obtein the related objects. · ICollection<TEntity> · IEnumerable<TEntity> · IEnumerable · IListSource For the previous interfaces, I wish recommend the following post from Jose Miguel Torres. Navigation properties allow us, to get and query easily objects related with the Entity. Methods? There is only one method in the Entity object. “Create{Entity}”, that allow us to create an object of the Entity by sending the parameters needed to create it. Finally After this chapter, we know what is an Entity, how is related to the DB and the relation to other Entities. In following chapters, we will se CRUD operations(Create, Read, Update, Delete).

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