OData: Entity data model

OData used Entity Data Model to describe the data exposed by an OData service.

Entity data model describes the structure of data, regardless of how it stored physically (For example, data can be stored in RDBMS, xls file, repositories, Hadoop system, file systems etc.,). Since data stored in different databases can follow different schema, EDM comes up with a generic approach by describing the data in the form of EDM.

Basic things to understand in EDM.

a.   Entities

b.   Entity Types

c.    Complex Types

d.   Entity Sets

e.   Dynamic Property

Entities

Entities are the instance of entity types. An entity type can be any class like Employee, Customer, Product etc.,

Entity Type

Entity type describes the structure of data with the Entity Data Model. Entity type define the properties and relationships of an entity. All the Entity types must have a key.

For example, EntityType for the vehicle Car is defined like below.

<EntityType Name="Car">
   <Key>
      <PropertyRef Name="Id" />
   </Key>
   <Property Name="Id" Type="Edm.Int16" />
   <Property Name="Model" Type="Edm.String" />
   <Property Name="ModelYear" Type="Edm.String" MaxLength="4" />
   <Property Name="Price" Type="Edm.Decimal" Scale="2" />
   <Property Name="Currency" Type="Edm.String" MaxLength="3" />
</EntityType>

One EntityType can refer other complex types (Which are not basic types like Edm.Int16, Edm.String etc.,). In the below example, Entity type ‘Manufacturer’ is referring Address entity type.

<EntityType Name="Manufacturer">
   <Key>
      <PropertyRef Name="Id" />
   </Key>
   <Property Name="Id" Type="Edm.Int16" />
   <Property Name="Name" Type="Edm.String" />
   <Property Name="Address" Type="olingo.odata.sample.Address" />
</EntityType>

<ComplexType Name="Address">
   <Property Name="Street" Type="Edm.String" />
   <Property Name="City" Type="Edm.String" />
   <Property Name="ZipCode" Type="Edm.String" />
   <Property Name="Country" Type="Edm.String" />
</ComplexType>

As you see, complex type does not have key.

In simple terms,

Entity type: A structured type with a key.

Complex type: A structured type without a key.

Entity Sets

Entity sets are the named collection of entities.

EntityKey

It is formed from a subset of properties of the EntityType. It is used to uniquely identify the entity type instances

For example, CustomerId is used to uniquely identify a customer entity.

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