Mapping

Entity Data Model Wizard^[31] in Visual Studio initially generates a one-to-one (1:1) mapping between the database schema and the conceptual schema in most of the cases. In the relational schema, the elements are composed of the tables, with the primary and foreign keys gluing the related tables together. In contrast, the Entity Types define the conceptual schema of the data.

The entity types are an aggregation of multiple typed fields – each field maps to a certain column in the database – and can contain information from multiple physical tables. The entity types can be related to each other, independent of the relationships in the physical schema. Related entities are also exposed similarly – via a field whose name denotes the relation they are participating in and accessing which, instead of retrieving the value from some column in the database, traverses the relationship and returns the entity (or a collection of entities) with which it is related.

Entity Types form the class of objects entities conform to, with the Entities being instances of the entity types. Entities represent individual objects that form a part of the problem being solved by the application and are indexed by a key. For example, converting the physical schema described above, we will have two entity types:

• CustomerEntity, which contains the customer's name from the Customers table, and the customer's address from the Contacts table.
• OrderEntity, which encapsulates the orders of a certain customer, retrieving it from the Orders table.

The logical schema and its mapping with the physical schema is represented as an Entity Data Model (EDM), specified as an XML file. ADO.NET Entity Framework uses the EDM to actually perform the mapping letting the application work with the entities, while internally abstracting the use of ADO.NET constructs like DataSet and RecordSet. ADO.NET Entity Framework performs the joins necessary to have entity reference information from multiple tables, or when a relationship is traversed. When an entity is updated, it traces back which table the information came from and issues SQL update statements to update the tables in which some data has been updated. ADO.NET Entity Framework uses eSQL, a derivative of SQL, to perform queries, set-theoretic operations, and updates on entities and their relationships. Queries in eSQL, if required, are then translated to the native SQL flavor of the underlying database.

Entity types and entity sets just form the logical EDM schema, and can be exposed as anything. ADO.NET Entity Framework includes Object Service that presents these entities as Objects with the elements and relationships exposed as properties. Thus Entity objects are just front-end to the instances of the EDM entity types, which lets Object Oriented languages access and use them. Similarly, other front-ends can be created, which expose the entities via web services (e.g., WCF Data Services) or XML that is used when entities are serialized for persistence storage or over-the-wire transfer.^[32]

Entities

Entities** are instances of EntityTypes; they represent the individual instances of the objects (such as customer, orders) to which the information pertains. The identity of an entity is defined by the entity type it is an instance of; in that sense an entity type defines the class an entity belongs to and also defines what properties an entity will have. Properties describe some aspect of the entity by giving it a name and a type. The properties of an entity type in ADO.NET Entity Framework are fully typed, and are fully compatible with the type system used in a DBMS system, as well as the Common Type System of the .NET Framework. A property can be SimpleType, or ComplexType, and can be multi-valued as well. All EntityTypes belong to some namespace, and have an EntityKey property that uniquely identifies each instance of the entity type. The different property types are distinguished as follows:

• SimpleType, corresponds to primitive data types such as Integer, Characters and Floating Point numbers.^[33]
• ComplexType, is an aggregate of multiple properties of type SimpleType, or ComplexType. Unlike EntityTypes, however, ComplexTypes cannot have an EntityKey. In Entity Framework v1 ComplexTypes cannot be inherited.^[34]

All entity instances are housed in EntityContainers, which are per-project containers for entities. Each project has one or more named EntityContainers, which can reference entities across multiple namespaces and entity types. Multiple instances of one entity type can be stored in collections called EntitySets. One entity type can have multiple EntitySets.

EDM primitive types (simple types):^[33][35]

Relationships

Any two entity types can be related, by either an Association relation or a Containment relation. For example, a shipment is billed to a customer is an association whereas an order contains order details is a containment relation. A containment relation can also be used to model inheritance between entities. The relation between two entity types is specified by a Relationship Type, instances of which, called Relationships, relate entity instances. In future releases, other kinds of relationship types such as Composition, or Identification, may be introduced.

Relationship types are characterized by their degree (arity) or the count of entity types they relate and their multiplicity. However, in the initial release of ADO.NET Entity Framework, relationships are limited to a binary (of degree two) bi-directional relationship. Multiplicity defines how many entity instances can be related together. Based on multiplicity, relationships can be either one-to-one, one-to-many, or many-to-many. Relationships between entities are named; the name is called a Role. It defines the purpose of the relationship.

A relationship type can also have an Operation or Action associated with it, which allows some action to be performed on an entity in the event of an action being performed on a related entity. A relationship can be specified to take an Action when some Operation is done on a related entity. For example, on deleting an entity that forms the part of a relation (the OnDelete operation) the actions that can be taken are:^[36]

• Cascade, which instructs to delete the relationship instance and all associated entity instances.
• None.

For association relationships, which can have different semantics at either ends, different actions can be specified for either end.

Schema definition language

ADO.NET Entity Framework uses an XML based Data Definition Language called Schema Definition Language (SDL) to define the EDM Schema. The SDL defines the SimpleTypes similar to the CTS primitive types, including String, Int32, Double, Decimal, Guid, and DateTime, among others. An Enumeration, which defines a map of primitive values and names, is also considered a simple type. Enumerations are supported from framework version 5.0 onwards only. ComplexTypes are created from an aggregation of other types. A collection of properties of these types define an Entity Type. This definition can be written in EBNF grammar as:

EntityType ::= 
  ENTITYTYPE entityTypeName [BASE entityTypeName]
    [ABSTRACT true|false] KEY propertyName [, propertyName]*
    {(propertyName PropertyType [PropertyFacet]*) +}

PropertyType ::= (
  (PrimitiveType [PrimitiveTypeFacets]*)
    | (complexTypeName)
    | RowType

  PropertyFacet ::= (
    [NULLABLE true | false]
    | [DEFAULT defaultVal] 
    | [MULTIPLICITY [1|*]]
  )

  PropertyTypeFacet ::= 
    MAXLENGTH | PRECISION | SCALE 
    | UNICODE | FIXEDLENGTH | COLLATION
    | DATETIMEKIND | PRESERVESECONDS

  PrimitiveType ::= 
    BINARY | STRING | BOOLEAN
    | SINGLE | DOUBLE | DECIMAL | GUID
    | BYTE | SBYTE | INT16 | INT32 | INT64
    | DATETIME | DATETIMEOFFSET | TIME
)

Facets are used to describe metadata of a property, such as whether it is nullable or has a default value, as also the cardinality of the property, i.e., whether the property is single valued or multi valued. A multiplicity of “1” denotes a single valued property; a “*” means it is a multi-valued property. As an example, an entity can be denoted in SDL as:^[37]

<ComplexType Name="Addr">
    <Property Name="Street" Type="String" Nullable="false" />
    <Property Name="City" Type="String" Nullable="false" />
    <Property Name="Country" Type="String" Nullable="false" />
    <Property Name="PostalCode" Type="Int32" />
</ComplexType>
<EntityType Name="Customer">
    <Key>
        <PropertyRef Name="Email" />
    </Key>
    <Property Name="Name" Type="String" />
    <Property Name="Email" Type="String" Nullable="false" />
    <Property Name="Address" Type="Addr" />
</EntityType>

A relationship type is defined as specifying the end points and their multiplicities. For example, a one-to-many relationship between Customer and Orders can be defined as

<Association Name="CustomerAndOrders">
    <End Type="Customer" Multiplicity="1" />
    <End Type="Orders" Multiplicity="*">
        <OnDelete Action="Cascade" />
    </End>
</Association>

Entity SQL

ADO.NET Entity Framework uses a variant of the Structured Query Language, named Entity SQL, which is aimed at writing declarative queries and updates over entities and entity relationships – at the conceptual level. It differs from SQL in that it does not have explicit constructs for joins because the EDM is designed to abstract partitioning data across tables.

Querying against the conceptual model is facilitated by EntityClient classes, which accepts an Entity SQL query. The query pipeline parses the Entity SQL query into a command tree, segregating the query across multiple tables, which is handed over to the EntityClient provider. Like ADO.NET data providers, an EntityClient provider is also initialized using a Connection object, which in addition to the usual parameters of data store and authentication info, requires the SDL schema and the mapping information. The EntityClient provider in turn then turns the Entity SQL command tree into an SQL query in the native flavor of the database. The execution of the query then returns an Entity SQL ResultSet, which is not limited to a tabular structure, unlike ADO.NET ResultSets.

Entity SQL enhances SQL by adding intrinsic support for:

• Types, as ADO.NET entities are fully typed.
• EntitySets, which are treated as collections of entities.
• Composability, which removes restrictions on where subqueries can be used.

LINQ to Entities

The LINQ to Entities provider allows LINQ to be used to query various RDBMS data sources. Several database server specific providers with Entity Framework support are available.

Native SQL

In the Entity Framework v4 new methods ExecuteStoreQuery() and ExecuteStoreCommand() were added to the class ObjectContext.

Visualizers

Visual Studio has a feature called Visualizer. A LINQ query written in Visual Studio can be viewed as Native SQL using a Visualizer during debug session. A Visualizer for LINQ to Entities (Object Query) targeting all RDBMS is available in the Visual Studio Marketplace.^[39]