从关联模型中选取

Question

I have two models, User and Post . Post belongs to User . User has a field titled name . I want to pluck the names of users. I was thinking of doing something like this.

some_cool_posts = Post.limit(5)

some_cool_posts.map(&:user).pluck(:name)

Unfortunately, pluck doesn't work with arrays so then I do

some_cool_posts.map(&:user).map(&:name)

I'm wondering is there a faster way to do this in Rails?

Answer 1

You have 6 requests to db, if use includes() then you will have two requests

some_cool_posts = Post.includes(:users).limit(5)
some_cool_posts.map(&:user).map(&:name)

And you can get posts with usernames, for one request, like:

some_cool_posts = Post.joins(:user).select('posts.*, users.name as user_name').limit(5)
some_cool_posts.map &:user_name

and, imho, the most faster way:

some_cool_posts = Post.limit(5)
uids = some_cool_posts.map &:user_id
User.where(id: uids).pluck :name

Answer 2

If I am understanding you correctly, you can also do:

@posts_users = some_cool_posts.collect {|post| post.user }
@users_names = @posts_users.collect { |user| user.name }

and you can try shortening it to this, but i'm not sure this will work:

@users_names = some_cool_posts.collect {|post| post.user }.collect { |user| user.name }