# Effective querying in belongs to associations

In this post i'll go through how to write efficient queries for __belongs_to__ associations. I primarly work with __Rails__ so it will focus on that a bit but i'll share the SQL versions too so you can implement similarly everywhere.

## Example domain model
Let's take 2 simple models.
![models](https://i.imgur.com/sjyim1V.png)

## Problem
Now we have to find all people who belong to a billable role(when billable is true basically).

## Solution
The simplest way is to use Ruby `Enumerable` method like
> `Person.all.select { |person| person.role.billable? }`

This will work but it's not optimal as it will hit the database for each person(could be millions). Also it gets a lot of useless data from the `role` table which we don't need.

Here what we want is for the database to do the heavy lifiting instead. We can use `join` here and filter out the role using `where` clause.

With Activerecord
> `Person.all.joins(:role).where(roles: { billable: true })`

which will generate a SQL like:
![sql](https://i.imgur.com/PcNlxST.png)

Here we have succesfuly used out `belongs_to` association by writing a good SQL query. These are very small and simple things one must keep in mind to keep their Rails or any other app fast and robust.