Scope Hunter

A RuboCop extension that catches duplicate ActiveRecord queries and replaces them with the named scope you already wrote.

Rails lets you define reusable query logic as named scopes:

class User < ApplicationRecord
  scope :active, -> { where(status: :active) }
end

But over time, the same query gets copy-pasted elsewhere without anyone realising the scope already exists:

# In a controller
User.where(status: :active)          # duplicates User.active

# In a service object
User.where(status: :active).order(:name)  # also duplicates it

Scope Hunter finds these duplicates automatically and fixes them with rubocop -A.

1. Add to your Gemfile:

gem 'scope_hunter'

2. Add to .rubocop.yml:

require:
  - scope_hunter

ScopeHunter/UseExistingScope:
  Enabled: true

3. Run RuboCop:

bundle exec rubocop

That's it. Scope Hunter will scan your app/models/ directory and flag any query that matches a named scope.

Given this model:

class User < ApplicationRecord
  scope :active,    -> { where(status: :active) }
  scope :with_posts, -> { joins(:posts) }
  scope :recent,    -> { order(created_at: :desc) }
end

Scope Hunter flags these:

app/controllers/users_controller.rb:5:5: C: ScopeHunter/UseExistingScope:
  Query matches `User.active`. Use the scope instead.
  User.where(status: :active)
  ^^^^^^^^^^^^^^^^^^^^^^^^^^^

app/services/report.rb:12:5: C: ScopeHunter/UseExistingScope:
  Query matches `User.with_posts`. Use the scope instead.
  User.joins(:posts)
  ^^^^^^^^^^^^^^^^^^

Running rubocop -A autocorrects them:

# Before
User.where(status: :active).order(:name).limit(10)

# After — the matched part is replaced; the rest is kept
User.active.order(:name).limit(10)

• rewhere is treated as where — rewhere(status: :active) matches the same scope as where(status: :active)
• Hash key order doesn't matter — where(a: 1, b: 2) matches where(b: 2, a: 1)
• Parameterized scopes are matched by shape — scope :by_role, ->(r) { where(role: r) } matches any where(role: <value>)
• Trailing methods are preserved — .order(), .limit(), and anything after the matched query are kept intact
• Cross-file detection — scopes defined in app/models/ are detected even when the duplicate query is in a controller, service, or job
• Dynamic values are ignored — User.where(status: current_user.status) is never flagged because the runtime value is unknown

The cop ships with conservative autocorrect. Running rubocop -A will replace the flagged query with the scope name, preserving any chained methods that come after.

Add any of these to your .rubocop.yml under ScopeHunter/UseExistingScope:

ScopeHunter/UseExistingScope:
  Enabled: true

  # Glob patterns for model files to scan for scope definitions.
  # Defaults to app/models/**/*.rb
  ModelPaths:
    - "app/models/**/*.rb"
    - "app/domain/**/*.rb"   # add extra paths as needed

  # Models to skip entirely — useful for legacy or auto-generated models
  # where scope reuse isn't practical.
  IgnoreModels:
    - LegacyReport
    - AdminAuditLog

Controls which files are scanned for scope definitions. By default, Scope Hunter reads everything under app/models/. If your project keeps models elsewhere, add those paths here.

A list of model class names to exclude from both detection and indexing. Queries on these models are never flagged, and scopes inside them are never indexed.

scope :inactive, -> { where.not(status: :active) }

# Flagged
User.where.not(status: :active)

# Autocorrected to
User.inactive

The scope uses a lambda parameter — Scope Hunter matches by the key name, not the value.

scope :by_role, ->(role) { where(role: role) }

# Flagged — value :admin doesn't matter, the key `role:` matches
User.where(role: :admin)

# Autocorrected to
User.by_role

scope :active, -> { where(status: :active) }

# NOT flagged — runtime value is unknown, could be anything
User.where(status: current_status)
User.where(status: @status)
User.where(status: params[:status])

Scopes in your models are indexed once per run. You can write the query anywhere in your app:

# app/models/user.rb
class User < ApplicationRecord
  scope :active, -> { where(status: :active) }
end

# app/controllers/users_controller.rb
class UsersController < ApplicationController
  def index
    # Flagged — Scope Hunter found the matching scope in user.rb
    @users = User.where(status: :active)
  end
end

# .rubocop.yml
ScopeHunter/UseExistingScope:
  IgnoreModels:
    - LegacyReport

# Not flagged — LegacyReport is in the ignore list
LegacyReport.where(status: :active)

# Report all offenses
bundle exec rubocop

# Scan only your models directory
bundle exec rubocop app/models/

# Run only this cop
bundle exec rubocop --only ScopeHunter/UseExistingScope

# Autocorrect all offenses
bundle exec rubocop -A

# Autocorrect only this cop
bundle exec rubocop --only ScopeHunter/UseExistingScope -A

1. Index — Before checking any file, Scope Hunter reads every file matched by ModelPaths and builds an in-memory index of all named scopes, keyed by a normalized signature.
2. Normalize — Each scope's query chain is reduced to a canonical form: M=User|W={status:?}|J=[]|O=[]. Values are replaced with ? so the shape is matched, not the literal value.
3. Detect — For every ActiveRecord query found in the file being checked, the same normalization is applied and the result is looked up in the index.
4. Flag — If a match is found, an offense is reported. Queries with dynamic values are skipped before this step.
5. Autocorrect — The matched portion of the query is replaced with Model.scope_name; any trailing method chain is appended unchanged.

# Install dependencies
bundle install

# Run tests
bundle exec rspec

# Run tests with coverage report
bundle exec rspec --format documentation

# Install the gem locally
bundle exec rake install

1. Update lib/scope_hunter/version.rb
2. Run bundle exec rake release

This creates a git tag, pushes the commit and tag, and publishes the gem to rubygems.org.

Bug reports and pull requests are welcome at github.com/Ajithxolo/scope_hunter.

1. Fork the repository
2. Create a feature branch: git checkout -b my-feature
3. Add tests for your change (we target 90%+ coverage)
4. Make your change and confirm tests pass: bundle exec rspec
5. Push and open a pull request

MIT — see LICENSE.txt.