Field processors

Ramses supports Nefertari field processors. Ramses field processors also have access to Nefertari’s wrapper API which provides additional helpers.


To setup a field processor, you can define the _processors property in your field definition (same level as _db_settings). It should be an array of processor names to apply. You can also use the _backref_processors property to specify processors for backref field. For backref processors to work, _db_settings must contain the following properties: document, type=relationship and backref_name.

"username": {
    "_processors": ["lowercase"]

You can read more about processors in Nefertari’s field processors documentation including the list of keyword arguments passed to processors.


If we had following processors defined:

from .my_helpers import get_stories_by_ids

def lowercase(**kwargs):
    """ Make :new_value: lowercase """
    return (kwargs['new_value'] or '').lower()

def validate_stories_exist(**kwargs):
    """ Make sure added stories exist. """
    story_ids = kwargs['new_value']
    if story_ids:
        # Get stories by ids
        stories = get_stories_by_ids(story_ids)
        if not stories or len(stories) < len(story_ids):
            raise Exception("Some of provided stories do not exist")
    return story_ids
# User model json
    "type": "object",
    "title": "User schema",
    "$schema": "",
    "properties": {
        "stories": {
            "_db_settings": {
                "type": "relationship",
                "document": "Story",
                "backref_name": "owner"
            "_processors": ["validate_stories_exist"],
            "_backref_processors": ["lowercase"]
  • validate_stories_exist processor will be run when request changes User.stories value. The processor will make sure all of story IDs from request exist.
  • lowercase processor will be run when request changes Story.owner field. The processor will lowercase new value of the Story.owner field.