使用：has_many关联和多个条件查找

Question

In my application, Users view Pins which I store in a separate associations table called View. The table has an ID, a pin_id, a user_id and a rank. I would like to find Pins which meet certain criteria which the user has not seen. I am using Rails 3.2.13.

Here are my model associations:

class Pin
has_many :views
has_many :users, :through => :views

class View
belongs_to :pin
belongs_to :user

class User
has_many :pins
has_many :views  
has_many :pins, :through => :views

Right now I am doing this, which works:

pins_controller.rb

b = TRUE
sex = current_user.sex
views = current_user.views
seen = views.map(&:pin_id)
@pins = Pin.find(:first, :conditions => ["sex = ? AND active = ? AND id not in (?)", sex, b, seen])

I thought it would be more elegant to use a join to pull the objects using a single database query. However, I cannot get this to work. I have read a bunch of different SO articles, but I have not been able to reproduce the above using a single query. Am I correct in thinking the above is roundabout and that there is a better way?

Answer 1

You should be able to just join up pins to view and view to users . I think the below will work, although would need the specific details on the view to users association:

Pin.joins(:views => :user).where("views.sex = ? and views.active = ? and users.id not in (?)", current_user.sex, true, [current_user.id])

Answer 2

Try squeel its wounderful and more better than normal active Record query methods and give you more powerful features

https://github.com/activerecord-hackery/squeel

http://railscasts.com/episodes/354-squeel

Answer 3

You could do something like this:

Pin.joins(:views).where(sex: sex, active: b).
  merge(View.where(View.arel_table[:user_id].not_eq(current_user.id))).first