How to create a relation to multiple assets in my Plugin Model/Record?
I think what you're asking is how to define the relationship using Active Record in the other direction - going from Craft's AssetFileRecord to your plugin's EstateRecord.

And the answer is that you can't without hacking the AssetFileRecord and manually adding a BELONGS_TO relation back to your Estate Record.

You're going to be much better off ditching records and active record for this all-together, I think.  Have your service layer use the Query Builder craft()-&gt;db-&gt;createCommand() directly, then you can join in any data you want from any tables outside of the context of active record.