Finding a Rails Model That Has All Associations

How to filter on a Rails model where all associations exist.

Rails: Find Models with ALL Associations

The Problem

For a project I’m working on, I was trying to build a check-box filter, where each check-box represents a relation in a has_many through association, and each new checked box further reduces the results.

For example, I’ve got a bunch of hospitals each with a set of features. Each check-box corresponds to a feature, and with each newly checked box, the number of hospitals returned should decrease, only return hospitals with ALL of the matching features.

I have an app with the following models:

  • Hospital
  • HospitalFeatures
  • Features

with HospitalFeatures being the join table between the two primary models. Here are the associations:

class Hospital < ActiveRecord::Base
  has_many :hospital_features
  has_many :features, through: :hospital_features
class HospitalFeature < ActiveRecord::Base
  belongs_to :hospital
  belongs_to :feature
class Feature < ActiveRecord::Base
  has_many :hospital_features
  has_many :hospitals, through: :hospital_features

I have a check-box form that lists all of the features available.

<%= label_tag("helipad", "Helipad") %>
<%= check_box_tag("features[]", "helipad") %>

<%= label_tag("telesurgery", "Telesurgery") %>
<%= check_box_tag("features[]", "telesurgery") %>

<%= label_tag("emergency room", "Emergency Room") %>
<%= check_box_tag("features[]", "emergency room") %>

The Solution

So if you check “helipad” and “telesurgery”, it should return only hospitals with both “helipad” AND “telesurgery” rather than any hospital with either “helipad” OR “telesurgery”.

When I set out to do this, I assumed it would be trivial in Rails. Turned out not to be the case. Here’s how I got it done.

First you will need to join your tables…

filtered_hospitals = Hospital.joins(:features)

At this point, you’ve just got a big table with everything in it.

So you filter for where the feature name is in the array of searched features:

filtered_hospitals = Hospital.joins(:features)
  .where(' in (?)', searched_features)

Where searched_features is something like ["telesurgery", "helipad"].

So if you want 2 features, and a hospital has both, you will get 2 copies of that hospital. So you can group on

filtered_hospitals = Hospital.joins(:features)
  .where(' in (?)', searched_features)

At this point, you will have all the hospitals that had ANY of the searched features rather than ALL of them. To fix that, you make sure that COUNT(*) is equal to the number of searched features. The final query being:

filtered_hospitals = Hospital.joins(:features)
  .where(' in (?)', searched_features)
  .having('COUNT(*) = ?', searched_features.length)

And there you have it. filtered_hospitals will be only the hospitals that have an association for every one of the desired features.

My Original Attempts

If you’re still reading, that means you didn’t just copy and paste that last snippet of code and want to actually learn why more obvious methods might not work. That’s awesome.

First Go

The move obvious and simplest attempt would be something like:

filtered_hospitals = Hospital.joins(:features)
  .where( features: {name: searched_features} )

This does the same join as the final solution, but looks for features where is in searched_features. It’s the same as:

filtered_hospitals = Hospital.joins(:features)
  .where( ' in (?)', searched_features )

This does the exact thing I was trying to avoid, which returns any hospital that has ANY searched feature. So adding features increases your number of results rather than narrowing it down.

Second Go - Bad SQL

My next attempt was to work with somewhat more raw SQL. I’d build a query string to evaluate based on each feature in the searched_features list.

query_string = (" = ? AND " * searched_features.length)[0..-6]

This just repeats “ = ? AND” as many times as there are features, and then removes the final AND. Not the cleanest, admittedly. Then I would run the following query:

filtered_hospitals = Hospital.joins(:features)
  .where(query_string, searched_features)

This just doesn’t work at all. When you join the hospitals and features table, you end up with multiple rows of the same hospital. One for each associated feature. This query would look for a single row, with all of the features. That’s obviously impossible as each row for a given hospital can only have one feature. Bad attempt.


This is just a query that seems like it would be very common and like Rails would have a built-in query for just this. Turns out that’s not the case and that it’s actually harder to find a clear solution than I expected. Once you see it though, it makes perfect sense and I think this will be a useful pattern to remember down the road.