[BUG] `Document.save` failed if the `_id` field is a customized field

[amos402]

Describe the bug
Document.save failed to create a new doc if the _id field is a customized field.
The save implementation changed in 9e89d30 cause it.

To Reproduce

class TestUser(Document):
    id: str
    name: str = Field(..., type=str, alias="_id")


async def test_model_insert():
    user = TestUser.model_construct()
    user.name = "test"
    await user.save()

pymongo.errors.OperationFailure: Plan executor error during findAndModify :: caused by :: Performing an update on the path '_id' would modify the immutable field '_id', full error: {'ok': 0.0, 'errmsg': "Plan executor error during findAndModify :: caused by :: Performing an update on the path '_id' would modify the immutable field '_id'", 'code': 66, 'codeName': 'ImmutableField'}

Expected behavior
The API should work as it said insert it if it does not yet exist.

Additional context
Add any other context about the problem here.

[Roman Right]

This is unexpected way to use models. Internally Beanie uses id as the _id field. I'll check if this can be adjusted to support your use case. For now please use id as the _id field

[github-actions]

This issue is stale because it has been open 30 days with no activity.

[github-actions]

This issue was closed because it has been stalled for 14 days with no activity.