S3 | Model Extensions | Component Resources

Component Resources

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 (qualified 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 Methods

When using a link-table to link a primary resource to it's components, the RESTful methods can handle this in a number of different ways, depending on what is desired:

• 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 over link actuation in S3CRUD:

Link Actuation Method	replace	hide	link	embed
CRUD Method
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:

s3mgr.model.add_component("my_component",               # Tablename of the component
                          my_master=dict(               # Tablename of the master table
                              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 method (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 not use this widget or a widget validator for the right key constraint!

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