I backup a database called RatePosts from production server and restored it on my development machine. When I tried to access the diagram I received the following message:
Database diagram support objects cannot be installed because this database does not have a valid owner. To continue, first use the Files page of theDatabase Properties dialog box or the ALTER AUTHORIZATION statement to setthe database owner to a valid login, then add the database diagram support objects.
To solve that, open a new query and run query below
ALTER AUTHORIZATION ON DATABASE::DatabaseName TO [ComputerName\Administrator];
In my case, my database name: RatePosts, machine name: AppDev05
ALTER AUTHORIZATION ON DATABASE::RatePosts TO [AppDev05\Administrator];
Done. Problem solved.