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[wiki:S3] | [wiki:S3XRC/ModelExtensions Model Extensions] | Component Resources
= Component Resources =

[[Image(ComponentTypes.png)]]

== 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 Methods ===

When using a link-table to link a primary resource to its components, the RESTful methods can handle this in a number of different ways, depending on the CRUD method and the configuration setting:
  - ''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:

||= CRUD Method =||||||||= Link Actuation Method =||
||= =||='''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 embedded^2^) ||
||='''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 link^1^ || deletes the link^1^ ||
||='''list'''=|| list view of component || list view of link || list view of link ||  list view of link (with component embedded^2^) ||

  * ^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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