Super-Entities

Introduction

Sometimes it is useful to link the same component to multiple master entities without introducing a separate foreign key constraint for each master.

Example: both warehouses and offices have contact persons. Instead of creating separate warehouse_contact and office_contact components, you can make use of the super-entity concept to link a single site_contact component to both entities.

To achieve this, S3 supports a special type of key tables called Super-Entities (referring to the fact that they constitue a generalization concept):

A super-entity stores (tablename, uuid) pairs which identify records across multiple tables, and the primary key of the super-entity (super-key) can then be used for foreign key constraints to link components. The primary key of the super-entity is usually referred to as super-key, and the respective foreign key constraints are called super-link.

The super-entity concept is implemented in S3Model, and supported by both S3CRUD and S3Import. Custom methods can make use of the current.s3db API to handle super-entities as described below.

API

To simplify the handling of super-entities in the model, S3Model provides a unified API for super-entities:

Defining a Super-Entity

A super-entity can be defined using the super_entity() method of S3Model:

class MyModel(S3Model):

    def model(self):

        # Define a dict with nice names for the instance types:
        my_instance_types = {
            "my_instance_tablename": current.T("Nice name for the instance type"),
        }

        # Define the super-entity
        tablename = "my_super_entity"
        table = self.super_entity(tablename, 
                                  "my_super_id",                   # specify the super ID
                                  my_instance_types,               # specify the instance types
                                  Field("start_date", "datetime"), # shared field
                                  location_id())                   # shared field

Shared fields will mirror the values from the respective instance record, which allows easy access without need to join multiple tables.

Defining an Instance of a Super-Entity

To make a table an instance of a super-entity, you can use s3db.configure():

s3db.configure(table, super_entity = db.sit_situation)

By default, all fields that the table and the super-entity have in common will be mirrored in the super-entity ("shared fields"). You can override this by specifying a list of fields to be mirrored by the super-entity.

s3db.configure(table,
               super_entity = db.sit_situation,
               sit_situation_fields = ["datetime"])

In case your table uses different names for the shared fields, you can use a dict instead to specify a mapping:

s3db.configure(table,
               super_entity = db.sit_situation,
               sit_situation_fields = dict(datetime="timestmp", location_id="location_id"))

Linking to a Super-Entity

Both, instance tables as well as shared components need to be linked to the super-entity. This can be done by inserting a super-key field into the instance/component table which must have the same name as the primary key of the super-entity. You can use the super_link() function as a DRY method for this:

tablename = "pr_presence"
table = db.define_table(tablename,
                        super_link("pe_id", "pr_pentity"),
                        super_link("sit_id", "sit_situation"), # sit_id
                        ...)

Note: super_link generates a Field instance - if you just need the name of the super-key field, you can use:

sk = s3db.super_key(db.pr_pentity) # returns the string "pe_id"

Updating a Super-Entity

If you use s3_rest_controller() for CRUD, it will automatically create, update and delete super-entities as necessary.

Otherwise, or if you manipulate records outside s3_rest_controller, you have to update all super-entities which an instance table implements whenever you create, update or delete a record in that table.

To do this, you can use:

s3db.update_super(table, record)

where table is the instance table and record the newly created/updated record.

In case you're going to delete a record from an instance table, you may use:

s3db.delete_super(table, record)

before you delete the record, where table is the instance table and record the record to be deleted.

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