Not sure if it's currently possible, but would be nice to be able to define a "custom join" maybe.
Say we have a table Metadata and that table has PK Id and columns RowId, TableName and Data.
We also have tables Blog and Post.

In RowId we write down PK of the table with name in TableName. It would then be nice if we could explain to EF how to join from our tables Blog and Post to table Metadata.

If we have List<Metadata> Metadatas in Blog, we should be able to define in ModelBuilder that when we access Blog.Metadatas it would join by Blog.Id = Metadata.RowId and Metadata.TableName = 'Blog'.

I guess there is an issue with what happens if you want to add a Metadata entity to Metadatas list on Blog or Post 🤷