Similar to an entity, a relationship can also be associated with optional attributes that are derived from the metadata.
For example, from the `Membership` metadata aspect shown below, we’re able to derive the `HasMember` relationship that links a specific `Group` to a specific `User`. We can also include additional attribute to the relationship, e.g. importance, which corresponds to the position of the specific member in the original membership array. This allows complex graph query that travel only relationships that match certain criteria, e.g. "returns only the top-5 most important members of this group."
Similar to the entity attributes, relationship attributes should only be added based on the expected query patterns to reduce the indexing cost.
Relationships are meant to be "entity-neutral". In other words, one would expect to use the same `OwnedBy` relationship to link a `Dataset` to a `User` and to link a `Dashboard` to a `User`. As Pegasus doesn’t allow typing a field using multiple URNs (because they’re all essentially strings), we resort to using generic URN type for the source and destination.
We also introduce a `@pairings` [annotation](https://linkedin.github.io/rest.li/pdl_migration#shorthand-for-custom-properties) to limit the allowed source and destination URN types.
While it’s possible to model relationships in rest.li as [association resources](https://linkedin.github.io/rest.li/modeling/modeling#association), which often get stored as mapping tables, it is far more common to model them as "foreign keys" field in a metadata aspect. For instance, the `Ownership` aspect is likely to contain an array of owner’s corpuser URNs.
For one, the actual direction doesn’t really impact the execution of graph queries. Most graph DBs are fully capable of traversing edges in reverse direction efficiently.
That being said, generally there’s a more "natural way" to specify the direction of a relationship, which closely relate to how the metadata is stored. For example, the membership information for an LDAP group is generally stored as a list in group’s metadata. As a result, it’s more natural to model a `HasMember` relationship that points from a group to a member, instead of a `IsMemberOf` relationship pointing from member to group.
Since all relationships are explicitly declared, it’s fairly easy for a user to discover what relationships are available and their directionality by inspecting
the [relationships directory](../../metadata-models/src/main/pegasus/com/linkedin/metadata/relationship). It’s also possible to provide a UI for the catalog of entities and relationships for analysts who are interested in building complex graph queries to gain insights into the metadata.
