How to run gitlab pipelines on merge request to main

I want to implement a workflow in gitlab that runs when a pr is raised to the main branch. I used the below script but it is not working as expected. Also, I want (I want it to compare the current feature branch with the main branch for changes, is it possible?).

lint_files:
  before_script:
    - export CI_DEFAULT_BRANCH=$(git remote show origin | grep 'HEAD branch' | cut -d' ' -f5)
    - apt-get update && apt-get install -y git
  image: python:3.10
  tags:
    - gitlab-org-docker
  stage: merge_request_checks
  script: 
    - echo 'linting models'
    - echo "source default name - $CI_DEFAULT_BRANCH"
    - git fetch --depth=1 origin $CI_MERGE_REQUEST_TARGET_BRANCH_NAME:$CI_MERGE_REQUEST_TARGET_BRANCH_NAME
    - git checkout $CI_MERGE_REQUEST_TARGET_BRANCH_NAME
    - pip install sqlfluff==1.3.2
    - CHANGED_FILES=$(git diff --name-only ${CI_DEFAULT_BRANCH} ${CI_COMMIT_REF_NAME} | grep '^models/')
    - |
      for file in $CHANGED_FILES
      do
        echo "Linting $file"
        sqlfluff lint "$file"
      done
  rules:
    - if: $CI_PIPELINE_SOURCE == "merge_request_event"
      changes: 
        paths:
          - 'models/*'
    - if: '$CI_COMMIT_REF_NAME == "production"'
      when: never

I want this job to be triggered when a merge request is raised to main branch and there are changes in models/ folder. I have issues with both of these conditions. I am unable to trigger the job for prs to main. And the changes is checking for the current commit but not the whole feature branch. Can anyone help me to achieve the required functionality.

Also, the above script fails with the error which needs to be fixed in addition to above two conditions:
fatal: ambiguous argument 'main': unknown revision or path not in the working tree.
Use '--' to separate paths from revisions, like this:
'git <command> [<revision>...] -- [<file>...]'

I think the syntax is different.

Try changing the rules like the following:

rules:
    - if: '$CI_MERGE_REQUEST_TARGET_BRANCH_NAME == "main" && $CI_MERGE_REQUEST_SOURCE_BRANCH_NAME =~ /^feature-/'
      changes:
        - my-folder/**/*