This commit adds support for two settings:
FOREIGNTYPE - A field that is used to store the type of the owner.
POLYMORPHIC - A shortcut to set FOREIGNKEY and FOREIGNTYPE to the same
value suffixed by "Id" and "Type" respectively.
The type is stored as the table name, which I thought might be useful
for other queries.
The biggest gotcha of this commit is that I flipped the definition of
has_one and belongs_to. gorm is very flexible such that it didn't
really care if it was a has_one or belongs_to, and can pretty much
determine it at runtime. For the sake of the error, I had to define
one of them as belongs_to, and I chose the one with the fields as
the belongs_to, like ActiveRecord. The error could probably be
genericized to "gorm cannot determine type", but I think it's nicer
to tell people DONT DO PATTERN XYZ CAUSE IT WONT WORK. Functionally,
it doesn't matter.
It looks like gorm always uses the snake form of a column by
convention, as seen by searching DBName in scope.go. These counts
were erroring out without the ToSnake'd foreign keys.
Further, the code for has_many and has_one becomes the same (which
makes sense), so I combined the two cases.
Jinzhu