Use case

Use case

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Let?s take the example of a blog with the following models:

  • A post habtm tags
  • A tag habtm posts

habtm stands for has_and_belongs_to_many.

how is it represented in rails ?

Let?s assume that the Post and Tag models already exist.

This is the migration for the join table

And then the associations in models

Cool! That works!

Few months later, you want to order tags in each post for searching purposes.

HABTM to has_many :through

Why ?

In our case, has_many :through allows us to declare the join table as model. And so, in order to operate (filter, sort, allow, reject, etc..) at a join table level. By doing this, we?ll be able to store the rank of a tag in the post’s tag list directly in the join model.

Migration issue

A model needs a primary key to be retrieved. By convention rails uses the id column as primary key. The problem is that we’ve removed the id column from our join table (id: false). this makes sense because we just need the post_id and tag_id to retrieve a record.

But in order to use has_many :through, we need to add a primary key to the posts_tags table. So how to do so ?

Then in models

Voil !

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