wiki:S3/S3Model

Version 31 (modified by Fran Boon, 8 years ago) ( diff )

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S3Model

Purpose

The S3Model class defines the framework for data models in Eden. It extends the web2py concept by:

  • implicit and lazy model loading
  • an extensible, per-table configuration pattern
  • a meta-model for projected entities (the so-called "resource component model")
  • a meta-model for multi-table keys (the so-called "super-entity model")

Data models can be implemented as subclasses of S3Model, and then loaded on demand - with automatic resolution of cross-model dependencies. This saves processing time as it will always only load those models which are actually needed to process the request.

Concepts

Dynamic Model Loading

While web2py executes all models during every request cycle, S3Model only executes the models needed to process the request.

That means though that all names (e.g. tables, variables, functions) which are defined in S3Models need to be requested explicitly before they can be used. To facilitate that, a global S3Model instance s3db serves as model loader.

In web2py, database tables can be accessed from the global db object:

table = db.my_table

In S3, tables must be requested from s3db:

table = s3db.my_table

Model loading happens only when a name from this model is requested, i.e. s3db loads no models whatsoever unless names are requested. If the model defining the table (or function, variable etc) has not yet been executed during the request, s3db will do so at this point.

S3Models can request other S3Models to be loaded, thereby automatically resolving dependencies. Note that for models where the Eden module is deactivated in the current template, s3db runs the defaults()-method instead of the model()-method (except for mandatory modules). If dependencies between S3Models are circular (i.e. X requesting Y and Y requesting X), S3Model will raise an exception.

It is wise to split Eden modules into multiple S3Models - so that dynamic model loading always only loads the necessary minimum for a requested name. However, too excessive splitting can increase loader overheads and the risk of bugs, so tables which are used together most of the time should be kept in the same S3Model.

Defining Models

Modules

S3 data models reside in modules/s3db.

The file name of each Python module in modules/s3db corresponds to the Eden module prefix. All names with this prefix will be looked up from this file.

Example:

  • Tablename: org_office => Module Prefix: "org" => Looked up from: modules/s3db/org.py

All S3 data models need to be imported in models/00_tables.py:

import s3db.assess
import s3db.asset
import s3db.auth
import s3db.cap
...

Every Python module in modules/s3db must have an __all__ statement declaring the classes and functions to be imported:

__all__ = ["S3DeploymentModel",
           "S3DeploymentAlertModel",
           "deploy_rheader",
           "deploy_apply",
           "deploy_alert_select_recipients",
           "deploy_response_select_mission",
           ]

Important: Undeclared classes or functions are not available to the model loader!

All names in __all__ starting with the module prefix (e.g. deploy_) can be accessed globally with current.s3db.<name> (e.g. current.s3db.deploy_apply), without need to import them explicitly.

The modules/s3db/skeleton.py module is a well-commented skeleton module to explain how things should look like inside an S3 model.

Note: it is possible (and sometimes even recommendable) to split an s3db module into sub-modules, and then import the names from the sub-modules into the main module.

Model Classes

Every data model is defined as a subclass of S3Model:

class S3DeploymentModel(S3Model):

names

All names from the model which shall be globally accessible (i.e. tables, functions, variables, classes) must be declared in the names class variable:

class S3DeploymentModel(S3Model):

    names = ["deploy_event_type",
             "deploy_mission",
             "deploy_mission_id",
             "deploy_mission_document",
             "deploy_application",
             "deploy_assignment",
             "deploy_assignment_appraisal",
             "deploy_assignment_experience",
             ]

These names can then be accessed via current.s3db.<name> (e.g. current.s3db.deploy_mission_id).

Important: All table names and names which are returned from a model class must use the module prefix (otherwise they can't be found).

model() function

Every S3Model subclass must implement the model() function. This function defines all tables, functions and variables of the model:

class S3DeploymentModel(S3Model):

    ...

    def model(self):

To define a table, the model() function must use self.define_table (instead of current.db.define_table):

    def model(self):

        tablename = "deploy_mission"
        table = self.define_table(tablename,
                                  ...)

The model function must return a dict with the definitions of all names as declared in the names class-variable (except table names):

class MyModel(S3Model):

    names = ["my_function", "my_variable"]

    def model(self):

        my_variable = "example"

        return dict(my_own_function = self.my_function
                    my_variable = my_variable
                   )

    @staticmethod
    def my function():

      ...
      return

Ideally, custom functions in model classes which are returned from model() should be declared @staticmethod or @classmethod to allow the instance to be garbage-collected (i.e. release the thread-global pointer to the instance from current.s3db).

defaults() function

Every model class should define a defaults() function which returns safe defaults for the declared names in case the Eden module has been disabled per deployment-settings.

This is particularly important for re-usable fields holding foreign keys to tables defined in this model:

class S3DeploymentModel(S3Model):

    names = [...
             "deploy_mission_id",
             ]

    def model(self):

        ...

        mission_id = S3ReusableField("mission_id", table,
                                     ...
                                     )

        return dict(deploy_mission_id = mission_id)

    def defaults(self):

        # Module disabled, define a safe default for "mission_id":
        mission_id = S3ReusableField("mission_id", "integer", readable=False, writable=False)
        return dict(deploy_mission_id = mission_id)

Defining additional functions

The S3Model base class implements a number of useful helper functions to implement models, among others:

  • super_entity and super_link to define or reference super entities
  • add_components to define resource components
  • configure to define table configuration settings

These functions should not be overwritten in the subclass.

To avoid name collisions, names of additional functions (i.e. besides model() and defaults()) should always start with the module prefix:

class MyModel(S3Model):

   def model(self):
       ...

   def defaults(self):

   # Use the module prefix in function names to avoid accidental overrides of superclass functions:
   def my_additional_function(self, param1, param2):
       ...

current.s3db

current.s3db is a global, empty instance of the S3Model class that loads tables, functions and variables from other models on demand.

Loading Tables, Functions and Variables from Models

current.s3db allows easy access to names using the attribute-notation:

Loading a table:

table = current.s3db.my_table

Loading a function:

my_function = current.s3db.my_function

If you have the name in a variable, you can use the item-notation instead:

tablename = "my_table"
table = current.s3db[tablename]

Note: The attribute- or item-notations will raise an AttributeError if the name can not be found (e.g. when the respective module is disabled).

To avoid exceptions due to disabled modules, one can use the table() function to access tables:

# Returns None if "my_table" is not found
table = current.s3db.table("my_table")

Note: s3db.table() will also return functions or variables with the specified name.

To limit the lookup to tables, use db_only:

# Returns only tables, but not functions or variables
table = current.s3db.table("my_table", db_only=True)

To only lookup functions and variables, but not tables, you can use the get() function:

# Returns only functions or variables, but not tables
my_function = current.s3db.get("my_function")

Table Configuration Settings

S3Model provides a key-value store for each table which is used to store per-table configuration settings.

The following functions can be used to manage table configuration settings:

Function Use-Case
configure(<tablename>, <key>=<value>, <key>=<value>, ...)to add or change configuration settings for a table
get_config(<tablename>, <key>, <default>)to retrieve a configuration setting
clear_config(<tablename>, <key>)to remove a configuration setting for a table

Note: In get_config(), the <default> parameter is optional (default: None), it will be returned if <key> is not configured for this table.

Note: These functions are available both in S3Model instances (self.* or current.s3db.*) and as class methods (S3Model.*).

Important: get_config() does not load the respective model class. Settings configured inside the model class are therefore only available outside of it (e.g. with s3db.get_config()) after the model has been loaded (e.g. with s3db.<tablename>)!

Example:

# Configure list_fields for the org_organisation table
s3db.configure("org_organisation", 
               list_fields=["id",
                            "name", 
                            "acronym", 
                            "organisation_type_id", 
                            "country", 
                            "website"])

See Also

Constructing Resources

current.s3db also provides a method to define a resource (see S3/S3Resource):

# Define a resource
resource = current.s3db.resource("my_table")

# Limit to certain record IDs
resource = current.s3db.resource("my_table", id=[1, 3, 7])

# Use a filter
from s3 import S3FieldSelector
resource = current.s3db.resource("my_table", filter=S3FieldSelector("id").belongs([1, 3, 7]))

# Apply a URL filter
resource = current.s3db.resource("my_table", vars={"~.id__belongs": "1, 3, 7"})

Components

Components are sub-tables of a master entity ("has"-relationship).

Components can be defined in S3Models using the add_components method, e.g.

# Offices (org_office table) are a component of organisations (org_organisation table),
# linked through the organisation_id field in org_office
current.s3db.add_components("org_organisation", org_office="organisation_id")

Read more here: Component Resources

Super-Entities

Super-entities are key tables spanning multiple entities. This allows references to multiple entities using a single foreign key.

S3Model provides a number of functions to define and manipulate super entities.

Read more here: Super-Entities


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