What is a GitLab line_code as referenced when creating a new merge request thread

Question

I'm trying to create a discussion note on a merge request on a certain line of a file with the GitLab api using this endpoint: https://docs.gitlab.com/ee/api/discussions.html#create-new-merge-request-thread

Part of the payload asks for a line_code

Attribute	Type	Required	Description
position[line_range][start][line_code]	string	yes	Line code for the start line

When I issue a POST I get a response with:

    "message": "400 (Bad request) \"Note {:line_code=>[\"can't be blank\", \"must be a valid line code\"], :position=>[\"is incomplete\"]}\" not given"

What is this line_code? Is it some kind of calculated value? The documentation is rather vague here.

When I issue a GET for all the current notes on a merge_request I can see some notes have this line_code (see below). I'm trying to figure out how to create that value for new notes.

 {
        "id": 89,
        "type": "DiffNote",
        "body": "4",
        "attachment": null,
        "author": {
            "id": 6,
            "name": "brian c",
            "username": "bc",
            "state": "active",
            "avatar_url": "https://www.gravatar.com/avatar/f590a9cf57136732dd0cb5z9b1563390?s=80&d=identicon",
            "web_url": "http://gitlab.mycompany.us/thisIsMe"
        },
        "created_at": "2021-01-11T21:46:23.861Z",
        "updated_at": "2021-01-11T21:46:23.861Z",
        "system": false,
        "noteable_id": 21,
        "noteable_type": "MergeRequest",
        "position": {
            "base_sha": "3bf8094f0d54fc70a66698bd582f25c77243de3b",
            "start_sha": "3bf8094f0d54fc70a66698bd582f25c77243de3b",
            "head_sha": "a10e73cf84eae38286df56f4b58fa221d7eefc44",
            "old_path": "b.txt",
            "new_path": "b.txt",
            "position_type": "text",
            "old_line": null,
            "new_line": 4,
            "line_range": {
                "start": {
                    "line_code": "aceba96ffdf13ce4cd4171c0248420cc03108ef0_0_4",
                    "type": "new",
                    "old_line": null,
                    "new_line": 4
                },
                "end": {
                    "line_code": "aceba96ffdf13ce4cd4171c0248420cc03108ef0_0_4",
                    "type": "new",
                    "old_line": null,
                    "new_line": 4
                }
            }
        },
        "resolvable": true,
        "resolved": false,
        "resolved_by": null,
        "confidential": false,
        "noteable_iid": 3,
        "commands_changes": {}
    },

Answer 1

Line code is hash of the file name + underscore + old line number + underscore + new line number

The documentation is wrong. line_code is required only if you are using position.line_range which is only required for adding diff note spanning multiple lines of diff. You don't need to deal with line_code for single-line diff notes. So it is not a required parameter. You can just use position.old_line or position.new_line.

Answer 2

That represents the line in the file you want the comment to appear on. For Merge Requests, comments can either be on the code or general discussions (though the API names seem to be backwards).

To add a general discussion comment, you can use the Notes API: https://docs.gitlab.com/ee/api/notes.html#create-new-merge-request-note. This will look like the comment here: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/52673#note_495396729

To add a comment to the changed code in a Merge Request, you can use the Discussions API here: https://docs.gitlab.com/ee/api/discussions.html#create-new-merge-request-thread. This operation has options to set the file path and line number a comment should start on, a range that a comment applies to, etc. This is an example of a comment on the code itself: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/52673/diffs#2eda52c44979de93f257b305ada778372eacba0b_6_5