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A key serves as a unique identifier for each entity instance. Most entities in EF have a single key, which maps to the concept of a primary key in relational databases (for entities without keys, see Keyless entities). Entities can have additional keys beyond the primary key (see Alternate Keys for more information).

By convention, a property named Id or <type name>Id will be configured as the primary key of an entity.

Note

Owned entity types use different rules to define keys.

You can configure a single property to be the primary key of an entity as follows:

Entity Framework Tutorial Key. The Key attribute can be applied to a property to make it a key property in an entity class and the corresponding column to a primary key column in the database. The default Code First convention look for a property named 'Id', or a combination of 'class name' and 'Id', such as BookId. Oct 23, 2016 We’re going to make use of Entity Framework Designer, which is included as part of Visual Studio, to create our model. Project - Add New Item Select Data from the left menu and then ADO.NET Entity Data Model. Enter BloggingModel as the name and click OK. This launches the Entity Data Model Wizard. Select Generate from Database and click Next.

You can also configure multiple properties to be the key of an entity - this is known as a composite key. Composite keys can only be configured using the Fluent API; conventions will never setup a composite key, and you can not use Data Annotations to configure one.

Primary key name

By convention, on relational databases primary keys are created with the name PK_<type name>. You can configure the name of the primary key constraint as follows:

Key types and values

While EF Core supports using properties of any primitive type as the primary key, including string, Guid, byte[] and others, not all databases support all types as keys. In some cases the key values can be converted to a supported type automatically, otherwise the conversion should be specified manually.

Key properties must always have a non-default value when adding a new entity to the context, but some types will be generated by the database. In that case EF will try to generate a temporary value when the entity is added for tracking purposes. After SaveChanges is called the temporary value will be replaced by the value generated by the database.

Important

If a key property has its value generated by the database and a non-default value is specified when an entity is added, then EF will assume that the entity already exists in the database and will try to update it instead of inserting a new one. To avoid this turn off value generation or see how to specify explicit values for generated properties.

Alternate Keys

An alternate key serves as an alternate unique identifier for each entity instance in addition to the primary key; it can be used as the target of a relationship. When using a relational database this maps to the concept of a unique index/constraint on the alternate key column(s) and one or more foreign key constraints that reference the column(s).

Tip

If you just want to enforce uniqueness on a column, define a unique index rather than an alternate key (see Indexes). In EF, alternate keys are read-only and provide additional semantics over unique indexes because they can be used as the target of a foreign key.

Alternate keys are typically introduced for you when needed and you do not need to manually configure them. By convention, an alternate key is introduced for you when you identify a property which isn't the primary key as the target of a relationship.

You can also configure a single property to be an alternate key:

You can also configure multiple properties to be an alternate key (known as a composite alternate key):

Finally, by convention, the index and constraint that are introduced for an alternate key will be named AK_<type name>_<property name> (for composite alternate keys <property name> becomes an underscore separated list of property names). You can configure the name of the alternate key's index and unique constraint:

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This video and step-by-step walkthrough provide an introduction to Database First development using Entity Framework. Database First allows you to reverse engineer a model from an existing database. The model is stored in an EDMX file (.edmx extension) and can be viewed and edited in the Entity Framework Designer. The classes that you interact with in your application are automatically generated from the EDMX file.

Watch the video

Entity Framework Database First

This video provides an introduction to Database First development using Entity Framework. Database First allows you to reverse engineer a model from an existing database. The model is stored in an EDMX file (.edmx extension) and can be viewed and edited in the Entity Framework Designer. The classes that you interact with in your application are automatically generated from the EDMX file.

Presented By: Rowan Miller

/xcom-enemy-unknown-cd-key-generator.html. Video: WMV MP4 WMV (ZIP)

Pre-Requisites

You will need to have at least Visual Studio 2010 or Visual Studio 2012 installed to complete this walkthrough.

If you are using Visual Studio 2010, you will also need to have NuGet installed.

1. Create an Existing Database

Typically when you are targeting an existing database it will already be created, but for this walkthrough we need to create a database to access.

The database server that is installed with Visual Studio is different depending on the version of Visual Studio you have installed:

• If you are using Visual Studio 2010 you'll be creating a SQL Express database.
• If you are using Visual Studio 2012 then you'll be creating a LocalDB database.

Let's go ahead and generate the database.

• Open Visual Studio

• View -> Server Explorer

• Right click on Data Connections -> Add Connection…

• If you haven’t connected to a database from Server Explorer before you’ll need to select Microsoft SQL Server as the data source

• Connect to either LocalDB or SQL Express, depending on which one you have installed, and enter DatabaseFirst.Blogging as the database name

• Select OK and you will be asked if you want to create a new database, select Yes

• The new database will now appear in Server Explorer, right-click on it and select New Query

• Copy the following SQL into the new query, then right-click on the query and select Execute

2. Create the Application

To keep things simple we’re going to build a basic console application that uses the Database First to perform data access:

• Open Visual Studio
• File -> New -> Project…
• Select Windows from the left menu and Console Application
• Enter DatabaseFirstSample as the name
• Select OK

3. Reverse Engineer Model

We’re going to make use of Entity Framework Designer, which is included as part of Visual Studio, to create our model.

Database Generated Key Entity Framework Examples

• Project -> Add New Item…

• Select Data from the left menu and then ADO.NET Entity Data Model

• Enter BloggingModel as the name and click OK

• This launches the Entity Data Model Wizard

• Select Generate from Database and click Next

• Select the connection to the database you created in the first section, enter BloggingContext as the name of the connection string and click Next

• Click the checkbox next to ‘Tables’ to import all tables and click ‘Finish’

Once the reverse engineer process completes the new model is added to your project and opened up for you to view in the Entity Framework Designer. An App.config file has also been added to your project with the connection details for the database.

Additional Steps in Visual Studio 2010

If you are working in Visual Studio 2010 there are some additional steps you need to follow to upgrade to the latest version of Entity Framework. Upgrading is important because it gives you access to an improved API surface, that is much easier to use, as well as the latest bug fixes.

First up, we need to get the latest version of Entity Framework from NuGet.

• Project –> Manage NuGet Packages…If you don’t have the Manage NuGet Packages… option you should install the latest version of NuGet
• Select the Online tab
• Select the EntityFramework package
• Click Install

Next, we need to swap our model to generate code that makes use of the DbContext API, which was introduced in later versions of Entity Framework.

• Right-click on an empty spot of your model in the EF Designer and select Add Code Generation Item…

• Select Online Templates from the left menu and search for DbContext

• Select the EF 5.x DbContext Generator for C#, enter BloggingModel as the name and click Add

4. Reading & Writing Data

Now that we have a model it’s time to use it to access some data. The classes we are going to use to access data are being automatically generated for you based on the EDMX file.

This screen shot is from Visual Studio 2012, if you are using Visual Studio 2010 the BloggingModel.tt and BloggingModel.Context.tt files will be directly under your project rather than nested under the EDMX file.

Implement the Main method in Program.cs as shown below. This code creates a new instance of our context and then uses it to insert a new Blog. Then it uses a LINQ query to retrieve all Blogs from the database ordered alphabetically by Title.

You can now run the application and test it out.

5. Dealing with Database Changes

Now it’s time to make some changes to our database schema, when we make these changes we also need to update our model to reflect those changes.

The first step is to make some changes to the database schema. We’re going to add a Users table to the schema.

• Right-click on the DatabaseFirst.Blogging database in Server Explorer and select New Query
• Copy the following SQL into the new query, then right-click on the query and select Execute

Entity Framework Recreate Database

Now that the schema is updated, it’s time to update the model with those changes.

• Right-click on an empty spot of your model in the EF Designer and select ‘Update Model from Database…’, this will launch the Update Wizard

• On the Add tab of the Update Wizard check the box next to Tables, this indicates that we want to add any new tables from the schema.The Refresh tab shows any existing tables in the model that will be checked for changes during the update. The Delete tabs show any tables that have been removed from the schema and will also be removed from the model as part of the update. The information on these two tabs is automatically detected and is provided for informational purposes only, you cannot change any settings.

• Click Finish on the Update Wizard

The model is now updated to include a new User entity that maps to the Users table we added to the database.

Summary

In this walkthrough we looked at Database First development, which allowed us to create a model in the EF Designer based on an existing database. We then used that model to read and write some data from the database. Finally, we updated the model to reflect changes we made to the database schema.