Database diagram support objects cannot be installed because this database does not have a valid owne

After I restored the database from the development server to stage server and try to add the diagram I got this error:

“Database diagram support objects cannot be installed because this database does not have a valid owner. To continue, first use the Files page of the Database Properties dialog box or the ALTER AUTHORIZATION statement to set the database owner to a valid login, then add the database diagram support objects”

Here is the solution for this:

1. Right Click on Database

2. Click Property

3. Click Files

4. Type "sa" without quote in Owner text box

5. Click OK

This will solve the problem