Add ActiveRecord::Relation#structurally_compatible?.

[kddnewton]

Summary

Adds a query method by which a user can tell if the relation that they're about to use for #or or #and is structurally compatible with the receiver.

Other Information

Previously you either needed to iterate over the values of the relation or just catch the ArgumentError thrown by #or or #and in order to determine if two relations were structurally compatible.

As for my use case: I have filters coming in from query parameters that are transformed into ActiveRecord::Relation objects. Those objects are then joined together using or and and to get to one single relation at the end. The issue is, if a developer adds a join into any of the transformations, then all of the sudden we've got structurally incompatible relations and some filters can't be used with others. What I'd like to do in this case is:

relations = [...]
relations.drop(1).inject(relations.first) do |current, other|
  if current.structurally_compatible?(other)
    current.or(other)
  else
    Model.where(id: current).or(Model.where(id: other))
  end
end

or something to that effect, so that I don't have to catch any errors and devs can use whatever grouping/joins they want to to get the filters working correctly.

[rails-bot]

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