Rely on through table name in has_many fixtures

[jpcody]

The fixtures code always builds join table names via the association's join_table method, which will use derive_join_table to build the table name via convention. But in the event a join table was created with a different table name, this will fall down. Using the through_association#table_name makes an affordance for both of these cases.

I think the existing code works from for the ReflectionProxy but should be overridden for the HasManyThroughProxy to handle if the table name is either overridden in the model or created with a different table name.

[wyaeld]

Some more background, we ran into this recently and came up with the same solution before finding @jpcody PR. We've ran the same code, and it will load has_many through fixtures correctly, as the HABTM does.

@tenderlove you had 2 commits relating to this 59ba85f & a9da99e
the problem is that the 2nd in the second one the HasManyThroughProxy doesn't implement join_table and therefore just inherits the ReflectionProxy implementation, which is based on Rails naming convention, and ignores the :through option

The test case with Parrot Treasure ParrotTreasure only tests HABTM.

If it was

class Parrot 
has_many :secrets
has_many :treasures, through: secrets

then you avoid the assumption on naming conventions

ping @senny as well just in case.

[tenderlove]

Oops! Makes sense.