Associations: DRY up the code which is generating conditions, and make it all use arel rather than SQL strings

Set the create scope to an empty hash in ThroughAssociationScope. For reasoning please see the inline code comments.

Use conditionals and implicit returns rather than explicit returns and postfix ifs (it's easier to read)

Remove has_cached_counter? and cached_counter_attribute_name from HasManyThroughAssociation, as the exact same methods are inherited from HasManyAssociation

Use the through association proxy for operations on the through record, so that those operations are automatically scoped and therefore construct_join_attributes does not need to use construct_owner_attributes.

Raise an error for associations which try to go :through a polymorphic association [#6212 state:resolved]

If a has_many goes :through a belongs_to, and the foreign key of the belongs_to changes, then the has_many should be considered stale.

Improved strategy for updating a belongs_to association when the foreign key changes. Rather than resetting each affected association when the foreign key changes, we should lazily check for 'staleness' (where fk does not match target id) when the association is accessed.

Revert "Optimize <association>_ids for hm:t with belongs_to source". The optimisation has too many edge cases, such as when the reflection, source reflection, or through reflection has conditions, orders, etc. [#6153 state:resolved]

This reverts commit 373b053d.

Conflicts:

	activerecord/lib/active_record/associations.rb

When a has_many association is not :uniq, appending the same record multiple times should append it to the @target multiple times [#5964 state:resolved]

Fix creation of has_many through records with custom primary_key option on belongs_to [#2990 state:resolved]

previously primary key value was always assigned nil which caused Oracle enhanced adapter failing tests