MissingIndexFindByChecker.md

MissingIndexFindByChecker finds columns that are used in find_by calls (including dynamic finders) but are missing a database index, which can lead to slow queries.

The checker uses Ruby's built-in Prism parser to scan project source files and detect the following patterns:

• find_by_<column>(value) — dynamic finder
• find_by(column: value) — symbol-key hash
• find_by("column" => value) — string-key hash

Note: This checker requires Ruby 3.3+ (where Prism is part of the standard library). It is automatically skipped on older Ruby versions.

The checker automatically skips:

• Primary key columns — always indexed by definition.
• Boolean columns — low-cardinality; single-column indexes on boolean columns are rarely useful and are typically ignored by the query planner.

Use case example:

class User < ApplicationRecord
end

User.find_by(email: "user@example.com")

If there is no index on the email column in the database, database_consistency will report it.

Message:

MissingIndexFindByChecker fail User email column is used in find_by but is missing an index (found at app/models/user.rb:42)