Next Generation Schema/Advanced Relationships Table Structure
Advanced Relationships Table Structure
The main table is link. It contains the begin and end date info, and the link_type foreign key field that specifies what kind of AR it is. It also has a count of how many other attributes that particular link has in the attribute_count field, and a created field that specifies when it was created.
There are tables for every possible combination of primary entities (artist, label, recording, release, release_group, url, work), all prefixed with l_ and all with the same format. Four of them are shown in the diagram above. They contain a field, modpending that is a count of pending changes to the AR, and three foreign keys: link that points back to the associated entry in the link table, and entity0 and entity1 that point to the associated entry in the corresponding primary entity table (i.e. artist, recording, url, etc.).
Each AR attribute either applies to a particular AR or it doesn't. The link_attribute table stores this information, having a record for each attribute_type (a foreign key field for link_attribute_type) of each link.
The attributes are themselves defined in the link_attribute_type table. Attributes form a number of trees (the vast majority of them are individual musical instruments). This tree structure is expressed with the parent, root and child_order fields; parent is the id of the parent attribute, or null if it's at the root; root is the root of the tree the attribute is part of, and child_order orders the children of a given parent attribute. Attributes also have names and descriptions which appear various places they are displayed, as well as gids (TODO: why?) and a last_updated timestamp.
The link_type_attribute_type table specifies what attributes can be applied to particular types of ARs; it has the necessary foreign key fields (link_type and attribute_type) and it also specifies how many instances of the attribute (or one of it's children) can be added to the particular AR type in the min and max fields. Currently, most of them allow the attributes to merely be present or absent, a few allow any number of copies of the attribute, or none. The "creative commons licensed download" attribute has to be included exactly once, while the instrument attribute (of the instrument type AR), requires at least one instance.