Database diagram support objects cannot be installed because this database does not have a valid owner. To continue, first use the Files page ...

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.