til/rails/find-records-with-multiple-associated-records.md

Find Records With Multiple Associated Records

Relational data often involves a table that has a one-to-many relationship with another table. For instance, a team can be made up of many members. A question we may want to ask of that data is, what are the records (teams) that are associated with more than one of this other table (members).

With a few SQL features that are supported by ActiveRecord's query syntax, we can answer that question.

To make it interesting, let's say we are trying to answer the question, "what are the teams that have multiple active members?"

Team
  .joins(:members)
  .where(members: { status: 'active' })
  .having("count(*) >= 2")
  .group("teams.id")
  .count

=> {
  123 => 2,
  345 => 3,
  567 => 2,
  ...
}

That final .count is going to manifest as a count(*) in the select clause. That count(*) aggregate combined with the group("teams.id") is going to flatten the results to be unique by team ID. Then the having clause will filter out all teams with a member count less than 2. And before that, the where will cut down the members to only those that are active.

If you just want the IDs, you can tack a #keys call onto the end of that query result.