在 Rails 中 has_and_belongs_to_many

Question

Is there anything explicitly wrong with using has_and_belongs_to_many associations in rails instead of has_many:through? I'm aware of thesearticles describing differences and work arounds, but they are from 2006. From things I've read on SO, it seems like people think that habtm is old and clunky, but what if a simple many to many join with no model necessary is what you're looking for?

Thoughts?

Answer 1

has_and_belongs_to_many is meant for simple many-to-many relationships.

has_many:through, on the other hand, is meant for indirect one-to-many relationships, or many-to-many relationships with properties.

If you're only looking for a simple many-to-many relationship, I can't see any reason not to use has_and_belongs_to_many.

Example many-to-many relationship:

User belongs to zero or more groups, and group has zero or more members (users).

Example many-to-many relationship with properties:

User belongs to zero or more groups, and group has zero or more members with ranks.

For example, Alice might be an Administrator in Group A, and a Moderator in Group B. You can hold this property in the join table.

Example indirect one-to-many relationship:

A category has zero or more sub-categories, and each sub-category has zero or more items.

A category therefore has zero or more items through its sub-categories.

Consider these categories:

Food → Fruits, Vegetables
Fruits → Apple, Orange, etc.
Vegetables → Carrot, Celery, etc.

therefore:

Food → Apple, Orange, Carrot, Celery, etc.

Answer 2

I think about it this way. Assuming you've already found that you need a many-to-many model:

X----1
  __/
 /
Y----2
  __/
 /  
Z----3

(x->1 y->1,2 z->2,3)

Use a HABTM relationship if you do NOT need to store information about each of the lines in my (hopefully recognizable) picture above.

If you need to store information about those lines (relationships), then use a "through".

So if you are just saying that people [XYZ] have and belong to projects [123] but do not need to say anything about person X on project 1, use a HABTM.

If you want to say that person X has project 1 and was assigned that project on a given date , you suddenly have a properly for that particular relationship and better use HMT.

Answer 3

There's nothing wrong with using has_and_belongs_to_many if you don't require a join model. I've just used it myself on a recent project.

Answer 4

I would never use HABTM not because of any concern about elegance but because I can always imagine wanting to add data to a relationship in the future even if I can't see the point now. Being lazy I would like to be able to just add the columns to the join rather than having to rework the relationships and then add the columns.