@ustimchik: consider adding this to your .rubocop.yml:

inherit_mode:
    merge:
        - Exclude
        - Include

AllCops:
    Include:
        - lib/path/to/your/engines/**/*.rb

See: https://rubocop.readthedocs.io/en/latest/configuration/#unusual-files-that-would-not-be-included-by-default

@ianyamey, it seems that this config somehow conflicts with rubocop-rails default config:

Finds 36 Rails/HasAndBelongsToMany offences

require:
  - ./linters/custom_cops.rb
  - rubocop-rails

inherit_mode:
  merge:
    - Exclude
    - Include

Still finds 36 Rails/HasAndBelongsToMany offences

require:
  - ./linters/custom_cops.rb
  - rubocop-rails

inherit_mode:
  merge:
    - Exclude
    - Include

AllCops:
  Include:
    - 'plugins/**/*.rb'

Only with specific rule overwrite it finds 50 Rails/HasAndBelongsToMany offenses and actually applies this rule to plugins/**/*.rb files

require:
  - ./linters/custom_cops.rb
  - rubocop-rails

inherit_mode:
  merge:
    - Exclude
    - Include

Rails/HasAndBelongsToMany:
  Include:
    - 'plugins/**/*.rb'

I'm having this issue as well. It may be more straightforward to include engine paths (ex. engines/**/app/models/**/*) for the relevant cops in default.yml, instead of adding new config features.

I am also having this issue, not with Engines but with Packwerk, which uses a similar subfolder structure - in my case it looks like packs/**/app/models/**/*. I'd like to apply the same rubocop config to all of my packs, but when I put this config in a root level .rubocop.yml, rubocop-rails doesn't apply the model/controller/etc-specific cops to any files within packs.

The only solution I've found so far is to copy my root-level .rubocop.yml into every single pack directory (packs/**/.rubocop.yml). This isn't sustainable and I would love to find a better option.