高效查询不同的计数和两列分组

Question

Given a simple model that consists of descriptions, tags, and some other fields

The results should be:

• a list of all tags in Entry.all without duplicates (eg Entry.select("DISTINCT(tag)") )

• the number of duplicates for each tag, also used to sort tags

• all descriptions for each tag sorted alphabetically, again without duplicates (however, the exactly same description can exist with a different tag)

Is it possible to combine this in one (efficient) query?

Edit:

def change
  create_table :entries do |t|
    t.datetime :datum, :null => false                            
    t.string :description                                        
    t.string :tag                                                
    (and some others)
  end

  add_index :entries, :user_id
end

Answer 1

It's better to create additional table:

rails g model Tag name:string description:string
rails g model Entry tag:references ...

And then just call them:

@entries = Entry.select('tag_id, count(tag_id) as total').group(:tag_id).includes(:tag)

After that, you will have all descriptions in your object:

@entries.first.tag.description # description of entry tag
@entries.first.tag.total # total number of such kind of tags

PS: Why just one tag per entry?