| RelationshipInterface | Relationship class for "has one" relationships |
| AbstractRelationship | Relationship class for "has one" relationships |
| BelongsTo | Relationship class for "belongs to" relationships |
| Exception | Relationships exception class |
| HasMany | Relationship class for "has many" relationships |
| HasOne | Relationship class for "has one" relationships |
| HasOneOf | Relationship class for "has one of" relationships |