The implementation of `attribute_method?` on Active Record requires
establishing a database connection and querying the schema. As a general
rule, we don't want to require database connections for any class macro,
as the class should be able to be loaded without a database (e.g. for
things like compiling assets).

Instead of eagerly defining these methods, we do it lazily the first
time they are accessed via `method_missing`. This should not cause any
performance hits, as it will only hit `method_missing` once for the
entire class.