Work with :through associations made easy

As explained in Rails association reference, defining a has_many association gives 17 methods.

We'll focus on the collection_singular_ids=(ids) part.

class Book has_many :book_genres has_many :genres, through: :book_genres end class Genre has_many :book_genres has_many :books, through: :book_genres end # the tie model class BookGenre belongs_to :book belongs_to :genre end 

Given a book, we'd get a genre_ids= (from has_many :genres). This method is very powerful. It's designed in such a way that you can use the value as-is straight from controller params in mass-assigment, and only the genres you specified would remain. Yes, genre_ids= will take care of identifying assocs being removed and new ones being added and only apply the change, not touching unchanged records!

book.genre_ids #=> [1, 2] # this will delete the BookGenre record tying this book to genre 1 # and create a new one tying it to genre 3, leaving tie to 2 unchanged! book.update!(genre_ids: ["2", "3"]) book.genre_ids #=> [2, 3]  

This only leaves validation errors. Say you want to validate that every book has at least one genre specified.

Easy, you can even use a macro validation:

Book.validates :genre_ids, length: { minimum: 1, message: :at_least_one_required } 

The last piece of the puzzle is how will the form react. You may need to align what field/helper you are using, so validation errors color the field nice and red on any problems.

<%= f.input :genre_ids %> # or maybe <%= f.association :genres %> 

Just make sure the param being submitted is :genre_ids, so mass-assignment works.

For StrongParams you may need to do this:

params.require(:book).permit( :title, genre_ids: [] ).