IDENTITY INSERT and LINQ to SQL

问题:

I have a SQL Server database. This database has a table called Item. Item has a property called "ID". ID is the primary key on my table. This primary key is an int with an increment value of 1. When I attempt to insert the record, I receive an error that says:

Cannot insert explicit value for identity column in table 'Item' when IDENTITY_INSERT is set to OFF.".

I am attempting to insert records using the following code:

  public int AddItem(Item i)
  {
    try
    {
      int id = 0;
      using (DatabaseContext context = new DatabaseContext())
      {
        i.CreatedOn = DateTime.UtcNow;
        context.Items.InsertOnSubmit(i);
        context.SubmitChanges();
        id = i.ID;
      }
      return id;
    }
    catch (Exception e)
    {
      LogException(e);
    }
  }

When I look at i.ID before submitting it, I notice that i.ID is set to 0. Which would imply that I'm trying to insert 0 as the identity. However, I'm not sure what it should be. Can someone help me out?

Thanks!

回答1:

It sounds simply as though your model is unaware of the fact that it is an identity; the ID column should be marked as db-generated (Auto Generated Value in the UI, or IsDbGenerated in the xml), and probably as the primary key. Then it will not attempt to insert it, and will update the value correctly after it is written.

回答2:

Set the "Auto Generated" property of the ID Property to "True".

回答3:

Check your ID property inside the Item class to ensure that it have attributes like this:

[Column(Storage="_ID", AutoSync=AutoSync.OnInsert,
    DbType="INT NOT NULL IDENTITY", IsPrimaryKey=true, IsDbGenerated=true)]

Look at the IsDbGenerated=true, it is the important guy here.

Maybe you created the DatabaseContext using the designer before adjusting the IDENTITY on the Sql Server, so just regenerate this class (by deleting the table in the designer and dropping it from the Server Explorer again).

回答4:

The ID will be zero until you call context.SubmitChanges();. Step through the code and look at the value after SubmitChanges has been invoked. Remember, the DB is actually assigning the ID (assuming that it's a auto-increment key).

Take a look at this:

Working with Entity Keys

回答5:

You must define in your mapping that id is generated in the database. The way to do this depends on the type of mapping you are using. if you are using code attributes make sure that you specify IsDbGenerated in the ColumnAttribute for the Id property (or in XML mapping). if you are using dbml file make sure that Auto Generated Value is set to true.

回答6:

The simplest way is: 1. open dbml 2. Select class for inserting 3. Select column with primary-key 4. check Auto Generated Property in properties window (it should be set to True)