Getting count by drilling down through relations

问题

I'm using Rails 4 and I want to get a count of votes for each individual post review for a given post.

Schema:

Post    PostReview    PostReviewVote
id      post_id       post_review_id
        user_id       user_id
                      voted_on_date

I have something like:

table.table.table-striped.table-hover
  thead
    tr
      th Post Name
      th Reviews Created For Post
      th Total Votes in All Reviews For Post
  tbody
    - for post in @posts
      tr
        td= post.name
        td= PostReview.where(post_id: post.id).size
        td= PostReview.where(post_id: post.id).PostReviewVotes.size

But it's not fetching, citing a runtime error:

undefined method `PostReviewVotes' for #<ActiveRecord::Relation []>

Any suggestions? My models have the proper associations, FYI.

回答1:

You could simply do this:

post.post_reviews.count
post.post_review_votes.count

If you have all the associations defined as you say.

or if you want a method for it...

In your Post model:

def post_votes_total
  self.post_review_votes.count
end

As long as you have defined the relationship in Post:

has_many :post_review_votes, through: :post_reviews

Since you are passing the @posts variable to the view :

post.post_votes_total

Using the built in association methods in the views is fine, but if logic gets more complicated you should really use a model method or a helper method.

回答2:

Couple of issues I see:

PostReview.where(post_id: post.id) can return more than one record.

Your relationship should be defined as post_review_votes, so you'd want to call .post_review_votes rather than PostReviewvotes

You might want one of the following: `PostReview.where(post_id: post.id).first.post_review_votes.size- If you only expect one PostReview this is probably what you want.

PostReview.where(post_id: post.id).collect{|pr| pr.post_review_votes.size}.sum - This will take all of the post reviews, get the size of their votes, and add them together.

Or pure SQL:

PostReviewVotes.includes(:post_review).where("post_reviews.post_id: ?", post.id).count

来源：https://stackoverflow.com/questions/21472347/getting-count-by-drilling-down-through-relations