如何基于多个关联模型查询模型

Question

I have a rails application where I have following models -

City, Hotel, Restaurant, Park.

Associations are like this -

class City < ActiveRecord::Base

 has_many :hotels
 has_many :restaurants
 has_many :parks

end

I want to find all cities that have at least one hotel or restaurant or Park.

How do I write single query to fetch such cities ?

Answer 1

For Rails 5, you can use like below

cities = City.includes(:hotels, :restaurants, :parks)
cities = ((cities.where.not(hotels: {id: nil})).or(cities.where.not(restaurants: {id: nil})).or(cities.where.not(parks: {id: nil})))

For lower version of rails , you need to use arel_table

Answer 2

The City model doesn't have any information about related stuff. You need to select the data from hotel/park/etc.

Use AR's includes to find the all Cities with specified relations.

City.includes(:hotels, :restaurants, :parks)

Answer 3

Most appropriate solution would be using counter cache

Then you should be able to query like

City.where('hotels_count > 0 OR restaurants_count > 0 OR parks_count > 0')

PS This query can be re-written many ways eg use .or method. Also, don't forget to reset cache counter if you have some data in associated tables.