{
  "errors": [
    {
      "extensions": {
        "path": "$.selectionSet.insert_a.args.objects[0]",
        "code": "constraint-violation"
      },
      "message": "Not-NULL violation. null value in column \"id\" violates not-null constraint"
    }
  ]
}

The following table definitions reproduce the above error:

Tables

Table a:

Table B:

Relationships

Table A:

Table B:

Invoking The Error

That error happens when inserting data from the "has one" side (table a), but not when inserting from the "belongs to" side (I wish Hasura could label relationships like this as well in addition to what's there to make things clear quickly):

the above error happens when you run this query:

mutation {
  insert_a(objects: {
    junk_a: "This is a's junk"
    b:{
      data: 
        {
          junk_b: "B's junk inserted when inserting a's junk"
        }
    }
  }){
    affected_rows
  }
}

but not when you run this query from the other side:

mutation {
  insert_b(objects: {
    junk_b: "This is B's junk"
    a:{
      data: 
        {
          junk_a: "A's junk inserted when inserting B's junk"
        }
    }
  }){
    affected_rows
  }
}

This could be related to #2508, but I am not sure cause what he is reporting on is different.