使用Rails和Mongoid的SQL到Mongodb

Question

I have the following document structure in Mongoid:

class Post
    include Mongoid::Document
    field "title", type: String
    field "body", type: String
    field "category_name", type: String
    field "category_id", type: Integer
end

I need to select all existing categories of posts to search. If this were SQL, I would:

SELECT distinct category_name, category_id FROM posts 

How would I do this in SQL query in Mongoid?

Answer 1

If you want the model to return only the category_name and category_id fields, use "only". Post.all.only(:category_name, :category_id) would return all posts of the db but only the values of those 2 attributes, all the other attributes would be nil.

But on Mongoid 3.1 you can do Post.distinct(:category_name) and it would return a list of the different category names of the posts as an array. On older versions of mongoid you can do Post.all.distinct(:category_name) to obtain the same return.