Add habtm/has_many :through support

[antoinematyja]

Hello 🙂

According to https://www.rubydoc.info/gems/activerecord/ActiveRecord/Reflection/AbstractReflection
"Countable" associations are :

• has_many
• has_and_belongs_to_many
• has_many :through

The current AssociationCountLoader works for has_many associations, but not for the other 2.

I added an implementation for habtm/has_many through which relies on joins with the association name (which should automatically go through all intermediate associations) + where with association name: primary key => array of primary keys (which should automatically create the where clause).

It also relies on an inverse_of definition, inspired by activerecord-precounter gem. I also added two errors (also inspired from the gem) in case of missing inverse_of, or in case of unhandled association.

Note: the activerecord-precounter gem also handles association scopes, which could be added to this loader.

[jamesbrooks]

Thanks for this @antoinematyja, I'll give it a test shortly but looks great - great for AssociationCountLoader to be able to handle as many relationship possibilities as it can :)