7. System-defined Taxonomy & Tags creation

Context

System-defined taxonomies are taxonomies created by the system. Some of these depend on Django settings (e.g. Languages) and others depends on a core data model (e.g. Organizations or Users). It is necessary to define how to create and validate the System-defined taxonomies and their tags.

Decision

System Tag lists and validation

Each Taxonomy has two methods for validating tags: #. validate_value #. validate_external_id

These functions will return True if a given tag is valid, based on its external ID or value. Subclasses should override these as needed, to implement different types of taxonomy behavior (e.g. based on a model or baed on Django settings).

For example validate_value("English") will return True for the language taxonomy if the English language is enabled in the Django settings. Likewise, validate_external_id("en") would return true, but validate_external_id("zz") would be False because there is no such language. Or, for a User taxonomy, validate_value("username") would return True if a user with that username exists, or validate_external_id(...) could validate if a user with that ID exists (note that the ID must be converted to a string).

In all of these cases, a Tag instance may or may not exist in the database. Before saving an ObjectTag which references a tag in these taxonomies, the tagging API will use either Taxonomy.tag_for_value or Taxonomy.tag_for_external_id. These methods are responsible for both validating the tag (like validate_...) but also auto-creating the Tag instance in case it doesn’t already exist. Subclasses should override these as needed.

In this way, the system-defined taxonomies are fully dynamic and can represent tags based on Languages, Users, or Organizations that may exist in large numbers or be constantly created.

At present, there isn’t a good way to list all of the potential tags that exist in a system-defined Taxonomy. We may add an API for that in the future, for example to list all of the available languages. However for other cases like users it doesn’t make sense to even try to list all of the available tags. So for now, the assumption is that the UI will not even try to display a list of available tags for system-defined taxonomies. After all, system-defined tags are usually applied automatically, rather than a user manually selecting from a list. If there is a need to show a list of tags to the user, use the API that lists the actually applied tags - i.e. the values of the ObjectTags currently applied to objects using the taxonomy.

Tags hard-coded by fixtures/migrations

In the future there may be system-defined taxonomies that are not dynamics at all, where the list of tags are defined by Tag instances created by a fixture or migration. However, as of now we don’t have a use case for that.