Trigger Gitlab pipeline on merge request into "main"

I have a Gitlab pipeline based on a .gitlab-ci.yml file. I want to trigger my pipeline when a merge request is created that merges any branch into the "main" branch. My current .gitlab-ci.yml code triggers the pipeline on any merge request or manual trigger and looks like this:

build:
  rules:
    - if: '$CI_PIPELINE_SOURCE == "merge_request_event"'
      when: always # triggers pipeline on merge request
    - if: '$CI_PIPELINE_SOURCE == "web"'
      when: always # triggers pipeline via web UI "Run pipeline" button
    - when: never # does not trigger pipeline on other events

But when I add a condition to the first "if:" to only trigger on a merge into "main" based on online examples, my pipeline does not trigger anymore. I tried two options:

Option 1:

build:
  rules:
    - if: '$CI_PIPELINE_SOURCE == "merge_request_event" && $CI_COMMIT_BRANCH == $CI_DEFAULT_BRANCH'
      when: always 
    - if: '$CI_PIPELINE_SOURCE == "web"'
      when: always 
    - when: never

Option 2:

build:
  rules:
    - if: '$CI_PIPELINE_SOURCE == "merge_request_event" && $CI_COMMIT_REF_NAME == "main"'
      when: always 
    - if: '$CI_PIPELINE_SOURCE == "web"'
      when: always 
    - when: never

The code is accepted and runs fine. But my pipeline does not trigger automatically anymore. Any help to answer my question is appreciated.

Solution

CI_COMMIT_BRANCH is not available as a predefined CI variable during a merge request. You can use MR specific predefined variables during an MR, here is example using CI_MERGE_REQUEST_TARGET_BRANCH_NAME

See more about predefined CI vars here

Option 1: Default branch (is not always main)

build:
  rules:
    - if: $CI_PIPELINE_SOURCE == "merge_request_event" && $CI_MERGE_REQUEST_TARGET_BRANCH_NAME == $CI_DEFAULT_BRANCH
      when: always 
    - when: never

Option 2: Explicit main

build:
  rules:
    - if: $CI_PIPELINE_SOURCE == "merge_request_event" && $CI_MERGE_REQUEST_TARGET_BRANCH_NAME == "main"
      when: always 
    - when: never