`=` does not type check

[christopherswenson]

= does not type check. I ran into this super weird case where I did join_one: something on something = field, when I meant to do join_one: something on something.field = field, and got a nasty SQL-generator error because it was confused that something was not scalar.

[mtoy-googly-moogly]

I am afraid to work on this because it might break the entire world and we don't have good tests. I agree it is a bug and at some point someone needs to write a comprehensive suite of tests so that it would be safe to fix this.