Practical .NET

Defining Columns, Primary, and Composite Keys in Entity Framework

There are two attributes you can put on your entity class properties: one is a convenience, the other is essential, and both are required when the primary key for a table consists of two columns.

When creating an entity Class to work with one of your tables in your Entity Framework model, you might find the Column attribute useful. For one purpose, it's a convenience: If you have a column called txtFirstName (because it follows some naming convention used by the database design team) and you'd prefer to have your property just called FirstName (because that's your coding convention), then the Column attribute will let you have the property name you want. Just decorate the property in your entity Class with the Column attribute to tie your FirstName property to your txtFirstName column, like this:

Public Class Customer
    <Column(Name := "txtFirstName")>
   Public Property FirstName() As String

And that's nice to have.

While used like this, the Column attribute is a convenience, the Key attribute is required -- you have to use it to tell Entity Framework which of your columns corresponds to your table's primary key. This example tells Entity Framework that the Id property is the primary key column for the table:

Public Class Customer
   <Key()>
   Public Property Id() As Boolean
   <Column(Name := "txtFirstName")>
   Public Property FirstName() As String

But what if the primary key for your table has two (or more) columns? It's not enough just to flag the columns using Key -- you also have to specify the order of the columns in the key. You'll need to use both the Key and the Column attributes to do that. This example:

Public Class Customer
   <Key>
   <Column(Order := 1)>
   Public Property FirstName As String
   <Key>
   <Column(Order := 2)>
   Public Property LastName As String

specifies that the Customer table has a composite key of FirstName and LastName with FirstName coming first.

About the Author

Peter Vogel is a system architect and principal in PH&V Information Services. PH&V provides full-stack consulting from UX design through object modeling to database design. Peter tweets about his VSM columns with the hashtag #vogelarticles. His blog posts on user experience design can be found at http://blog.learningtree.com/tag/ui/.

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