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S3 | Model Extensions | Component Resources
Component Resources
Table of Contents
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 S3Cube 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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