one-to-many through referance table - ruby-on-rails

Question

I'm trying to understand how to implement one-to-many relationship through reference table. I'm looking on this guide I though just write on one model has_many so it will be one-to-many but I'm not completely sure (I wrote something but it's not working). Anyway I'm doing this to save for me a table, and doing it right and not just working.
The model is as following:
Microposts, :id, :content
Tag, :id, :name
Tag_microposts, :tag_id, :micropost_id
Article, :id, :text
Article_microposts, :article_id, :micropost_id
I can do two microposts tables with the id of the tag/article. But I think doing it like this is better and righter.
In the end what's interesting me is to get microposts through tag model. So in the tag_controller be able to do:
def index
#tag = Tag.find(params[:id])
#microposts = #tag.microposts
end
Some code:
class Tag < ActiveRecord::Base
...
has_many :tag_microposts, foreign_key: :tag_id, dependent: :destroy
has_many :microposts, through: :tag_microposts, source: :micropost
...
end
class TagMicropost < ActiveRecord::Base
validates :tag_id, presence: true
validates :micropost_id, presence: true
end
class Micropost < ActiveRecord::Base
belongs_to :user
belongs_to :tag
validates :content, presence: true, length: {minimum: 10, maximum: 250}
validates :user_id, presence: true
end

Answer 1

May I ask why you are using a reference table for this? You can do a one-to-many association with only the two models you are associating. If you want to associate a tag with many posts you can just do this in your models.
class Tag < ActiveRecord::Base
has_many :microposts
end
class Micropost < ActiveRecord::Base
belongs_to :tag
#added in edit
belongs_to :article
validates :content, presence: true, length: {minimum: 10, maximum: 250}
validates :user_id, presence: true
end
This should let you do:
#tag.microposts
Just fine. The forgien key will be stored in your Micro post model so be sure to add the column. You can use an active migration for that. Call the column tag_id, rails should take care of the rest.
Edit*
A added the article association. The problem you raised is only relevant if you need to get the article/tag given the micropost. The code to do that is still pretty simple with this model.
#tag ||= #micropost.tag
Using the conditional assignment operator like this will only assign #tag if the association is there. If you give me more specifics about how these models will be used I can give you a better answer.