S3 | Model Extensions | Component Resources

Component Resources

Component resources are an S3 framework concept to simplify the implementation of and access to aggregation models, which is the most common type of data models used in Sahana-Eden.

Typically, such an aggregation model has a master entity and a number of associated (sub-)entities ("have"-relationships):

• organisations = have => offices, projects, teams
• persons = have => addresses, identities
• groups = have => members, tasks

Each of these relationships can be represented by a join between the respective tables.

The "Component Resource" extension provides the functionality to pre-configure and re-use these joins as pseudo-attributes ("components") of the master entity, thereby replacing the joins by simple identifiers (aliases) which can be used to construct queries, e.g. in URLs:

    # Join "office": org_organisation <-- id/organisation_id --> org_office

    /org/organisation/5/office

or

    # Join "contact": pr_person <-- pe_id/pe_id --> pr_contact

    /pr/person?contact.contact_method=EMAIL&contact.value=user@example.com

The "Component Resource" extension is one of the fundamental RAD concepts of the S3 framework.

Besides simplified construction of queries, views and self-processing forms it also allows implicit (=keyless) constraints in S3XML which simplifies the mapping and transformation from and to other data models using XSLT.

Overview

The following diagram describes the joins which are currently supported:

Note that, for efficiency reasons, "components" can not be nested in queries.

Link-Table Components

Components can be bound to their master records via link-tables. In such cases, the foreign key constraints for the component link are all in a separate link-table, whereas both the master table and the component table are completely independent:

  master (primary key) <==== (foreign key) link table (foreign key) ====> (primary key) component

Link-table component links have some advantages over simple foreign key constraints:

• they can carry attributes of their own (attributed link)
• they provide the option to bind the same component record to multiple master records (many-to-many)
• there are several different ways to actuate such links
• link-table links work both ways (i.e. with master/component exchanged, can be declared both ways at the same time)

However, they do have disadvantages too:

• overhead to maintain a separate database table (processing time, migration issues etc.)
• increased complexity to access and query resources (3 tables instead of 2)
• increased complexity to handle such links in CRUD and XML/XSLT

Link Actuation Options

The RESTful methods can handle the link-table in a number of different ways, depending on the CRUD method and the configured option:

• replace: hides the link table and always operates on the component table
• hide: hides the component table and always operates on the link table
• link: operates on the component table for single-record requests, and on the link table for summary requests (=without record ID) and delete
• embed: operates on the link table, embeds the component record in single-record requests

The following table gives an overview of link actuation in S3CRUD:

CRUD Method	Link Actuation Option
	replace	hide	link	embed
create	create-form for component	create-form for link	create-form for link	create-form for link with component embedded
read	read-view of component	read-view of link	read-view of component	read-view of link (with component embedded2)
update	update-form for component	update-form for link	update-form for component	update-form for link with component embedded
delete	deletes both, component and link	deletes the link	deletes the link1	deletes the link1
list	list view of component	list view of link	list view of link	list view of link (with component embedded2)

• ¹ = deletes the component together with the last link if autodelete option is set
• ² = not implemented yet

Other RESTful methods such as S3Search or S3Report may have their own definitions.

Declaration

The basic syntax of a link-table component link declaration is:

s3db.add_component("my_component",               # Tablename of the component
                   my_master=dict(               # Tablename of the master table
                       name="alias",             # Use this 'alias' as the component name
                       link="my_linktable",      # Tablename of the link table
                       joinby="fieldname",       # FK of the master table (=left key constraint)
                       key="fieldname",          # FK of the component table (=right key constraint)
                       actuate="replace",        # Actuation option (see above, optional, defaults to "link")
                       autodelete=False          # Delete the component record together with the last link (optional, default is False)
                       widget=Widget,            # Widget to use for embedding (optional, defaults to S3EmbedComponentWidget)
                       autocomplete="fieldname", # Field in the component to use for autocomplete when embedding the component
                   ))
                              

If no field is defined for autocomplete, then no autocomplete-widget will be used, but a standard SELECT of options for key (default behavior).

Important: if you specify a widget for embedding (e.g. S3AddPersonWidget), then you must ensure that the foreign key in the link-table doesn't also use either this widget or any other widget validator!

If name='alias' is specified, the component would not be addressed in the URL as master/component but as master/alias. This allows to link the same component table to the same master table more than once using different link tables.

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