Add Find Records With Multiple Associated Records as a Rails til

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

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+# 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?"
+
+```ruby
+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.