Fixes#1305
Before Rails 6 when the build_stubbed strategy assigned an id on a
record without a primary key column, the `id=` method would no-op, and
the record would end up looking like a new_record (i.e. `new_record?`
would return true because the id was nil).
This problem surfaced because of a [change in Rails 6][rails], which
caused `id=` to raise a potentially confusing
`ActiveModel::MissingAttributeError: can't write unknown attribute ''`
for records without a primary key column.
Since build_stubbed stubbed was calling `id=` for all instances,
regardless of whether they had primary keys, it was raising the above
error.
To avoid this error, we check whether the instance has a primary_key
defined before setting the id.
We also changed `persisted?` and `new_record?` to be less dependent on
the id. That way those methods will work as expected for records without
primary keys.
[rails]: b6828fc915
Co-authored-by: Jesse Bailey <jbailey117@gmail.com>
