Development: Creating a new model class tutorial

All model classes should inherit assembl.lib.sqla.BaseOps (as Base), and many will also inherit assembl.models.DiscussionBoundBase, which means that they exist within the context of a discussion. Classes that represent information that can be deleted while leaving a trace of their existence will also inherit assembl.lib.history_mixin.TombstonableMixin, and classes that can have a snapshot taken before modification will inherit assembl.lib.history_mixin.HistoryMixin.

We will take assembl.models.attachment.Document as a fairly typical example.

ORM mapping

The first step is to bind the class to a database table (__tablename__) and columns. Each column’s name is given by the class variable name, and various attributes are Column arguments, as described in Declarative Mapping.

class Document(DiscussionBoundBase):
    __tablename__ = "document"
    id = Column(Integer, primary_key=True)
    uri_id = Column(URLString)

If the class is a base class with subclasses, we declare a type (or sqla_type) column, using the pattern for joined table inheritance. We must define the identity for the base class in the __mapper_args__.

type = Column(String(60), nullable=False)
__mapper_args__ = {
    'polymorphic_identity': 'document',
    'polymorphic_on': 'type',
    'with_polymorphic': '*'

Subclasses will only define polymorphic_identity in their __mapper_args__.

When we create a foreign keys, we usually also define a relationship for that foreign key. That relationship may have a backref, which allows to traverse the class backwards. This is described in relationship configuration.

discussion_id = Column(Integer, ForeignKey(
    nullable=False, index=True)

discussion = relationship(
        cascade="all, delete-orphan"),

Note: The cascade options here are redundant; the cascade argument in the backref asks SQLAlchemy to handle the cascade, whereas the ondelete cascade ensures deletion by the database. This was done because we did not trust triggers in Virtuoso, but should be revisited with Postgres.

Note 2: TODO: It would be have been a good practice to put a comment in the Foreign key target to indicate the existence of the backref relationships, for documentation purposes.

Some more SQL arguments can be defined in the __table__args__ construct:

__table_args__ = (UniqueConstraint('discussion_id', 'uri_id'), )

DiscussionBoundBase protocol

Two methods have to be defined for subclasses of assembl.models.DiscussionBoundBase.

  1. assembl.models.DiscussionBoundBase.get_discussion_id() : Define how to get the discussion_id from an instance of the class. It may be given in a column of the instance or in a column of an associated instance. The object may have just been created, in which case the value may have been set either in the foreign, in the relationship, but maybe not both. It is thus good practice to look for both cases.
  2. assembl.models.DiscussionBoundBase.get_discussion_conditions(): a class method giving the SQLAlchemy expression that could be applied to the filter method to filter instances of this class belonging to the given discussion. This may involve joining on a relationship if the discussion_id is not a column of this class. In that case, use the provided alias_maker to obtain suitable aliases for joining. A good example is given in assembl.models.idea.IdeaLink.get_discussion_conditions(). But the case here is simple.
def get_discussion_id(self):
    return self.discussion_id or

def get_discussion_conditions(cls, discussion_id, alias_maker=None):
    return (cls.discussion_id == discussion_id,)

Extra collections

The traversal API (assembl.views.api2) relies on relationship introspection to find collections of objects related to any given instance. However, it is sometimes necessary to define collections of related objects that are not expressed by a relationship, or to define extra behaviour when an object is instantiated through this collection. These extra behaviours are defined in the class method extra_collections, as a dictionary of subclasses of assembl.views.traversal.AbstractCollectionDefinition, indexed by name. (collection definitions based on relationships are subclasses of assembl.views.traversal.CollectionDefinition.) See assembl.models.discussion.Discussion.extra_collections() for an example.

Duplicate handling

For some objects, uniqueness constraints can be set in the database, but it is more difficult in some cases, as the uniqueness constraints may span inheritance join tables. In that case, creation of an object will trigger the creation of a query that defines whether that object is unique. That query is created in assembl.lib.sqla.BaseOps.unique_query() and conditions are added in subclasses. The unique_query may not be enforcable for any given class, so the return value will mention whether to enforce it.

The query is used in assembl.lib.sqla.BaseOps.handle_duplication(), and if the object is found to be a duplicate of an object already stored in the database, what will happen depends on the value of the duplicate_handling parameter. Usually, this parameter is not set, and the classe’s default_duplicate_handling variable is consulted. The different ways duplicate are handled are defined in the assembl.lib.sqla.DuplicateHandling enum.

default_duplicate_handling = DuplicateHandling.USE_ORIGINAL

def unique_query(self):
    query, _ = super(Document, self).unique_query()
    return query.filter_by(uri_id=self.uri_id), True

CRUD permissions

Each class should define the permissions that are required to Create, Read, Update or Delete (CRUD) an instance of that class. These are expressed in the crud_permission class property, as in instance of assembl.auth.CrudPermissions.

crud_permissions = CrudPermissions(


Each class should define how it will be represented by default in the REST interfaces. This is done by creating an entry for that class in assembl/view_defs/default.json, as described in assembl.view_def. It is also useful to have an entry for the class in assembl/view_defs/changes.json, which determines how much data will be sent to the Websocket when the object is changed.

IMPORTANT: If you forget to do this, you cannot access instances of that class, but you will not know that through errrors or otherwise.

"Document": {
    "uri": "uri_id",
    "type": true,
    "discussion": true

Finally, create/update operations on the instance may only allow changing a subset of fields; this is defined in assembl/view_defs/default_reverse.json. Again, if the class is not defined here, the POST/PUT operations will fail, often silently.

"Document": {
    "uri": "uri_id",
    "type": false,
    "discussion": false


In the rare case where your class needs an __init__ method, and that constructor creates related objects as a side effect, make sure to include an explicit session= named argument in the constructor, so that those objects are created in the right database session. (e.g.: assembl.models.langstrings.LangStringEntry)

Migration and declaration

You need to create an Alembic migration adding the table of the new class to the database. As an example, Document class was created in assembl/alembic/versions/

Also, you should import the class in assembl/static/js/app/models/ so it is known to the application at startup.

Frontend model

You should create a backbone model and a backbone collection for that class, and add the new Model type to assembl/static/js/app/utils/types.js. (TODO: details.) Example in assembl/static/js/app/models/documents.js