GitHub GraphQL API list all custom fields in ProjectV2

Question

I'm using ProjectsV2 in Github, and since we don't have some pretty basic automations, I'm trying to write my own.

I need to get a list of all my custom fields in the project, so I can then use their ids to modify those fields using my script.

I already managed to get a list of projects, labels, issues, prs, but the custom fields are giving me grief.

I'm trying this and all sorts of variants (which worked for almost every other list I needed):

query { 
  repository(owner:"OWNER", name:"REPO") { 
    projectV2(number: 2) {
        ... on ProjectV2 {
          fields(last: 20) {
            nodes {
              ProjectV2Field
                nodes {
                id
                } 
            }
          }
        }
    }
  }}

What I get is:

{
"errors": [
    {
        "path": [
            "query",
            "repository",
            "projectV2",
            "... on ProjectV2",
            "fields",
            "nodes",
            "ProjectV2Field"
        ],
        "extensions": {
            "code": "selectionMismatch",
            "nodeName": "ProjectV2FieldConfiguration"
        },
        "locations": [
            {
                "line": 6,
                "column": 17
            }
        ],
        "message": "Selections can't be made directly on unions (see selections on ProjectV2FieldConfiguration)"
    },
    {
        "path": [
            "query",
            "repository",
            "projectV2",
            "... on ProjectV2",
            "fields",
            "nodes",
            "nodes"
        ],
        "extensions": {
            "code": "selectionMismatch",
            "nodeName": "ProjectV2FieldConfiguration"
        },
        "locations": [
            {
                "line": 6,
                "column": 17
            }
        ],
        "message": "Selections can't be made directly on unions (see selections on ProjectV2FieldConfiguration)"
    }
]

}

I am new to graphql and I am trying to read Github's docs on it but I just can't figure this structure or what the error message means.

Any tips that can point me on the right direction are very much appreciated. Thanks!

EDIT:

I made some progress. The following query gives me a list of (most of the) fields, but no single select ones:

query {
      repository(owner:"OWNER", name:"REPO") {
        projectV2(number: 2) {
            ... on ProjectV2 {
                fields(first: 100) {
                    nodes {
                        ... on ProjectV2Field {
                            id
                            name
                            dataType
                        }
                    }
                }
            }
        }
      }
}

Response (with ids redacted):

{
"data": {
    "repository": {
        "projectV2": {
            "fields": {
                "nodes": [
                    {
                        "id": "ID",
                        "name": "Title",
                        "dataType": "TITLE"
                    },
                    {
                        "id": "ID",
                        "name": "Assignees",
                        "dataType": "ASSIGNEES"
                    },
                    {},
                    {
                        "id": "ID",
                        "name": "Labels",
                        "dataType": "LABELS"
                    },
                    {
                        "id": "ID",
                        "name": "Linked pull requests",
                        "dataType": "LINKED_PULL_REQUESTS"
                    },
                    {
                        "id": "ID",
                        "name": "Reviewers",
                        "dataType": "REVIEWERS"
                    },
                    {
                        "id": "ID",
                        "name": "Repository",
                        "dataType": "REPOSITORY"
                    },
                    {
                        "id": "ID",
                        "name": "Milestone",
                        "dataType": "MILESTONE"
                    },
                    {
                        "id": "ID",
                        "name": "Due",
                        "dataType": "DATE"
                    },
                    {
                        "id": "ID",
                        "name": "Days",
                        "dataType": "NUMBER"
                    },
                    {
                        "id": "ID",
                        "name": "Target",
                        "dataType": "TEXT"
                    },
                    {
                        "id": "ID",
                        "name": "Start Date",
                        "dataType": "DATE"
                    },
                    {
                        "id": "ID",
                        "name": "End Date",
                        "dataType": "DATE"
                    },
                    {},
                    {
                        "id": "ID",
                        "name": "PR Size",
                        "dataType": "NUMBER"
                    },
                    {}
                ]
            }
        }
    }
}

}

Almost there!