Add a required option to singular associations

[sgrif]

In addition to defining the association, a required association will
also have its presence validated.

Before:

belongs_to :account
validates_presence_of :account

After:

belongs_to :account, required: true

This helps to draw a distinction between types of validations, since
validations on associations are generally for data integrity purposes,
and aren't usually set through form inputs.

[sgrif]

/cc @dhh

[dhh]

Looks great to me 👍!

[dhh]

Build failed?

[sgrif]

Issue with mysql and the test cases I added, updated, should pass on this run.

[guilleiguaran]

❤️

[claudiob]

@sgrif The documentation added by this PR says:

This will validate the association itself, not the id. You can use
+:inverse_of+ to avoid an extra query during validation.

This sentence is still part of the current documentation. The way in which I interpret this is that:

• calling belongs_to :user will make an extra query SELECT "users".* FROM "users" WHERE "users"."id" = ? LIMIT 1 [["id", 1]] to validate that the actual association is valid
• but calling belongs_to :user, inverse_of: false will not make that extra query

However that is not the case. The extra query still takes place since this code is still running:

if reflection.options[:required]
  model.validates_presence_of reflection.name
end

Can you help me understand if I'm missing something?
I'd like to use belongs_to and (if possible) not incur in that extra query, validating only the presence of user_id and not its association.