Deprecate delegating to `arel` in `Relation`

[kamipo]

Active Record doesn't rely delegating to arel in the internal since
425f2ca. The delegation is a lower priority than delegating to klass,
so it is pretty unclear which method is delegated to arel.

For example, bind_values method was removed at b06f64c (a series of
changes 79f71d3...b06f64c). But a
relation still could respond to the method because arel also have the
same named method (#28976).

Removing the delegation will achieve predictable behavior.

[kaspth]

Isn't arel still private API? So the delegation makes sense?

[rafaelfranca]

Right. Better to say that is going to be removed with no replacement. Also the next version of Rails after 5.2 is 6.0

[kaspth]

Great!

[kamipo]

Thanks!

[samstickland]

Not sure if this is the right place for this question, but I'm unsure how to move this to non-deprecated calls (short of manually generating SQL):

class TimesheetBlock
  scope :with_entries, -> {
    where(TimesheetEntry.where('timesheet_block_id = timesheet_blocks.id').exists)
  }
end

This generates SQL like this:

SELECT "timesheet_blocks".* FROM "timesheet_blocks" WHERE EXISTS (SELECT "timesheet_entries".* FROM "timesheet_entries" WHERE (timesheet_block_id = timesheet_blocks.id))

But gives this deprecation warning:

Delegating exists to arel is deprecated and will be removed in Rails 6.0

[kamipo]

Please use .arel.exists instead.

[samstickland]

@kamipo Thanks!