CKAN Coding Standards¶

Encapsulate SQLAlchemy in ckan.model¶

Ideally SQLAlchemy should only be used within ckan.model and not from other packages such as ckan.logic. For example instead of using an SQLAlchemy query from the logic package to retrieve a particular user from the database, we add a get() method to ckan.model.user.User:

@classmethod
def get(cls, user_id):
    query = ...
    .
    .
    .
    return query.first()

Now we can call this method from the logic package.

Database Migrations¶

When changes are made to the model classes in ckan.model that alter CKAN’s database schema, a migration script has to be added to migrate old CKAN databases to the new database schema when they upgrade their copies of CKAN. See Creating a new migration script.

Always go through the Action Functions¶

Whenever some code, for example in ckan.lib or ckan.controllers, wants to get, create, update or delete an object from CKAN’s model it should do so by calling a function from the ckan.logic.action package, and not by accessing ckan.model directly.

Action Functions are Exposed in the API¶

The functions in ckan.logic.action are exposed to the world as the The CKAN API. The API URL for an action function is automatically generated from the function name, for example ckan.logic.action.create.package_create() is exposed at /api/action/package_create. See Steve Yegge’s Google platforms rant for some interesting discussion about APIs.

All publicly visible functions in the ckan.logic.action.{create,delete,get,update} namespaces will be exposed through the The CKAN API. This includes functions imported by those modules, as well as any helper functions defined within those modules. To prevent inadvertent exposure of non-action functions through the action api, care should be taken to:

1. Import modules correctly (see Imports). For example:

   import ckan.lib.search as search
   
   search.query_for(...)
   

2. Hide any locally defined helper functions:

   def _a_useful_helper_function(x, y, z):
      '''This function is not exposed because it is marked as private```
      return x+y+z
   

3. Bring imported convenience functions into the module namespace as private members:

   _get_or_bust = logic.get_or_bust
   

Use get_action()¶

Don’t call logic.action functions directly, instead use get_action(). This allows plugins to override action functions using the IActions plugin interface. For example:

ckan.logic.get_action('group_activity_list_html')(...)

Instead of

ckan.logic.action.get.group_activity_list_html(...)

Auth Functions and check_access()¶

Each action function defined in ckan.logic.action should use its own corresponding auth function defined in ckan.logic.auth. Instead of calling its auth function directly, an action function should go through ckan.logic.check_access (which is aliased _check_access in the action modules) because this allows plugins to override auth functions using the IAuthFunctions plugin interface. For example:

def package_show(context, data_dict):
    _check_access('package_show', context, data_dict)

check_access will raise an exception if the user is not authorized, which the action function should not catch. When this happens the user will be shown an authorization error in their browser (or will receive one in their response from the API).

logic.get_or_bust()¶

The data_dict parameter of logic action functions may be user provided, so required files may be invalid or absent. Naive Code like:

id = data_dict['id']

may raise a KeyError and cause CKAN to crash with a 500 Server Error and no message to explain what went wrong. Instead do:

id = _get_or_bust(data_dict, "id")

which will raise ValidationError if "id" is not in data_dict. The ValidationError will be caught and the user will get a 400 Bad Request response and an error message explaining the problem.

Validation and ckan.logic.schema¶

Logic action functions can use schema defined in ckan.logic.schema to validate the contents of the data_dict parameters that users pass to them.

An action function should first check for a custom schema provided in the context, and failing that should retrieve its default schema directly, and then call _validate() to validate and convert the data. For example, here is the validation code from the user_create() action function:

schema = context.get('schema') or ckan.logic.schema.default_user_schema()
session = context['session']
validated_data_dict, errors = _validate(data_dict, schema, context)
if errors:
    session.rollback()
    raise ValidationError(errors)