Argument error could be more informative

[andrewcroome]

I recently managed to create a join table for image attachments on news items in which I failed to define a primary key like the below:

primary_key [:news_item_id, :asset_id]

The first sign of my mistake was rom-factory raising the following error when calling the relation's factory:

ArgumentError:
       ROM::Relation::Combined#by_pk arity is 1 (0 args given)

I was able to find my mistake with help from @timriley, but I wonder if we could add mention to that error to check one's primary key as a pointer to others?

The top of the stacktrace was:

/Users/andrewcroome/.rbenv/versions/2.4.2/lib/ruby/gems/2.4.0/gems/rom-core-4.2.0/lib/rom/relation/curried.rb:73:in `to_a'
/Users/andrewcroome/.rbenv/versions/2.4.2/lib/ruby/gems/2.4.0/gems/rom-core-4.2.0/lib/rom/relation/materializable.rb:55:in `first'
/Users/andrewcroome/.rbenv/versions/2.4.2/lib/ruby/gems/2.4.0/gems/rom-factory-0.6.0/lib/rom/factory/builder/persistable.rb:31:in `create'

[timriley]

Thinking we could raise our own error if pk in

rom-factory/lib/rom/factory/builder/persistable.rb

Line 29 in ee2462f

pk = primary_key_names.map { |key| persisted[key] }

is empty.