Squeel appears to be broken when defining an association with a where clause of type String, and a default_scope on one of the models.

I'm using rails 4.1.9, though I've seen it on other versions > 4 as well. Using squeel 1.2.3 and postgres.

Models

class Vehicle < ActiveRecord::Base
  # Specify a simple string based where clause on the association
  has_many :vehicle_assignments, -> { where("vehicles.id = 1") }
end

class VehicleAssignment < ActiveRecord::Base
  belongs_to :vehicle

  # Add a simple default scope
  default_scope lambda {
    where(id: 1)
  }
end

Code

 Vehicle.joins(:vehicle_assignments)

Result

PG::SyntaxError: ERROR:  syntax error at or near "="
LINE 1: ..."."id" AND "vehicle_assignments"."id" = 1, (vehicles.id = 1)

SQL

SELECT "vehicles".* FROM "vehicles" INNER JOIN "vehicle_assignments" ON
"vehicle_assignments"."vehicle_id" = "vehicles"."id" AND "vehicle_assignments"."id" = 1, (vehicles.id = 1)

As you can see, the last conditional is not being added to the SQL correctly. We expect AND (vehicles.id = 1) but get , (vehicles.id = 1). My assumption is that this occurs because vehicles.id = 1 is being added as a simple simple string. If we change the syntax to "vehicles.id" => 1 then everything works as expected.

This only occurs when there is both a string based where scope block in the association definition, and when the association's model has a default_scope. The error occurs even when not using squeel syntax anywhere in the app.

I've traced the problem to the collapse_wheres method, specifically in the final code block where String based where clauses are explicitly handled. String based where clauses are added to arel.where but they are never ANDed with the other groups. I'll add a pull request with what I think is a correct solution, though I will mention that I am not deeply versed in arel.

Any input would be greatly appreciated.