Part 5 - How to handle model changes in entity framework

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Part 2 - Entity Framework Model First Approach
Part 3 - Entity Framework Code First Approach
Part 4 - Customizing table & column names



In this video we will discuss, how to handle model changes after the database is already created. This is continuation to Part 4. Please watch Part 4 before proceeding.



At the moment the Employee class is as shown below

[Table("tblEmployees")]

public class Employee

{

    public int Id { get; set; }

    [Column("First_Name")]

    public string FirstName { get; set; }

    public string LastName { get; set; }

    public string Gender { get; set; }

    public int Salary { get; set; }

    public int DepartmentId { get; set; }

    [ForeignKey("DepartmentId")]

    public Department Department { get; set; }

}

The following is the table Entity Framework has generated based on the above Employee class


Now let us add JobTitle property to the Employee class. The modified Employee class is shown below.

[Table("tblEmployees")]

public class Employee

{

    public int Id { get; set; }

    [Column("First_Name")]

    public string FirstName { get; set; }

    public string LastName { get; set; }

    public string Gender { get; set; }

    public int Salary { get; set; }

    public int DepartmentId { get; set; }

    [ForeignKey("DepartmentId")]

    public Department Department { get; set; }

    public string JobTitle { get; set; }

}

At this point if we run the application, we get the following error. 
The model backing the 'EmployeeDBContext' context has changed since the database was created. Consider using Code First Migrations to update the database (http://go.microsoft.com/fwlink/?LinkId=238269).

This is because the model (i.e Employee class) has changed since the database was created. This means the Model and the database are no longer in sync and hence we get the error. To check if the model has changed since the database was created, entity framework uses __MigrationHistory table that is auto-generated.

To fix this error, we have to tell entity framework what to do when the model changes.

Add Global.asax file to the web application project. Include the following code in Application_Start() method. Here, we are telling the entity framework to drop and recreate database every time the model changes.

Database.SetInitializer(new DropCreateDatabaseIfModelChanges<EmployeeDBContext>());

Another option is, to drop and recreate the database always. To drop and recreate the database always we would change the code in Application_Start() method as shown below.

Database.SetInitializer(new DropCreateDatabaseAlways<EmployeeDBContext>());

Please Note: Database class is present in System.Data.Entity namespace.

Run the application, and notice that the database is dropped and recreated. But the webform does not display any data, as there is no data in Departments and tblEmployees tables. For now let's manually populate the tables using the followng SQL script.

Insert into Departments values ('IT', 'New York')

Insert into Departments values ('HR', 'London')

Insert into Departments values ('Payroll', 'Sydney')

Insert into tblEmployees values ('Mark', 'Hastings', 'Male', 60000, 1, 'Developer')

Insert into tblEmployees values ('Steve', 'Pound', 'Male', 45000, 3, 'Manager')

Insert into tblEmployees values ('Ben', 'Hoskins', 'Male', 70000, 1, 'Developer')

Insert into tblEmployees values ('Philip', 'Hastings', 'Male', 45000, 2, 'Recruiter')

Insert into tblEmployees values ('Mary', 'Lambeth', 'Female', 30000, 2, 'Recruiter')

Insert into tblEmployees values ('Valarie', 'Vikings', 'Female', 35000, 3, 'Manager')

Insert into tblEmployees values ('John', 'Stanmore', 'Male', 80000, 1, 'Developer')

Referesh the webform. Notice that JobTitle is not displayed on the WebForm. To fix this add a boundfield to the GridView control that displays Employees details as shown below.

<asp:GridView ID="GridView2" runat="server" AutoGenerateColumns="False"

                DataSource='<%# Eval("Employees") %>'>

    <Columns>

        <asp:BoundField DataField="FirstName" HeaderText="First Name" />

        <asp:BoundField DataField="LastName" HeaderText="Last Name" />

        <asp:BoundField DataField="Gender" HeaderText="Gender" />

        <asp:BoundField DataField="Salary" HeaderText="Salary" />

        <asp:BoundField DataField="JobTitle" HeaderText="Job Title" />

    </Columns>

</asp:GridView>