Active Record, how to make N-N relationship properly?

Question

I have a N-N relationship between the tables Notebook and Part. A Notebook has many Parts and a Part can be present in many Notebooks. Here's a example of the schema:

In other approach, as suggested by AJFaraday in comments, I would have a single join table notebook-parts with a role to identify if its a gpu, a processor, etc:

How can I use the approach 2 to invoke notebook.processor, notebook.gpu etc inside Rails? And also I'll need to have some empty parts since some notebooks doesn't have dedicated gpu's, or maybe have only ssd's without hd's etc ?

How the model's relationship's should be?

Answer 1

Just create a model for the join table and reference it in both models.

# models/notebook_part.rb
class NotebookPart
  belongs_to :notebook
  belongs_to :part
end

# models/part.rb
class Part
  has_many :notebook_parts
  has_many :notebooks, though: :notebook_parts
end

# models/notebook.rb
class Notebook
  has_many :notebook_parts
  has_many :parts, though: :notebook_parts
end

Then in your notebook model you can create methods based on part roles something like:

# models/notebook.rb
def processors
  parts.where(part_role: :processor)
end

Then make that for any other roles you deem necessary, or think of some metaprogramming loop to make those lists for you.