[Feature] [ActiveRecord] Proposal to support associations using array columns

[Carlos Ferreira da Silva]

Hey Everyone.

Last year I implemented array association on my GEM exclusively for PostgreSQL:

https://github.com/crashtech/torque-postgresql

https://github.com/crashtech/torque-postgresql/wiki/Has-Many

https://github.com/crashtech/torque-postgresql/wiki/Belongs-to-Many

I'm planning to push this feature to Active Record, so any database that supports array could use.

The main steps that I took was:

1. Allow `array` setting for `has_many`

2. Identify array associations with something like `connected_through_array?`

3. Create a generic method that infers the correct way to connect both keys (primary_key and foreign_key), instead of direct use of `table[key].eq(foreign_table[foreign_key]`

4. Allow `cast` to be called on Some Arel Nodes

5. Make sure that `AssociationScope` rely on the above generic method to build the condition

6. Create the correct `BindParam` for each value on an array association

7. Create the necessary `BelongsToManyReflection`, `BelongsToManyAssociation`, and `Builder::BelongsToMany`

8. Adjust `AutosaveAssociation` to behave properly against array associations

9. A couple of other minors adjusts to make it seamless to other associations

The only problem so far is that it doesn't work with Polymorphic, but, it's so far a big win, especially for systems with `tags` and `conversations` (with multiple users).

I was thinking about pushing it to Rails not only because it's a cool and handy feature, but also because one of the problems that I faced recently with the GEM, where the `eq` condition is hardcoded, and that annoys me since it could open a door for many other customized associations. Here are the commit and line that got me thinking about this:
https://github.com/rails/rails/commit/54de9b1e4f5823bf22001be60efcb996ce6d260b#diff-1663e5f930ac83fef235d7e2aa5edf65R185

I want to hear people's opinions. And, if I decided to move forward, I'll do one step at a time, probably starting by the generic method on step #3.