Database Relationships

Relationships - References, Sets and Collections

Although Redis is not a relational database like SQL, we still need to define relationships between our objects. A mail message has an author and recipients (all characters), a scene is attached to a room, etc. Ohm provides some built in concepts that let us manage these relationships.

References

References reflect a single relationship between two objects: A vehicle has a single driver. We can define a reference and then use it like any other database property:

class Vehicle < Ohm::Model
  reference :driver, "AresMUSH::Character"
end

in some command handler...
vehicle.update(driver: some_char)
client.emit "The driver is: #{vehicle.driver.name}."

For a 1-to-1 relationship, we may want to define the relationship going the other way as well:

class Character
  reference :vehicle, "AresMUSH::Vehicle"
end

in some command handler:
some_char.update(vehicle: some_vehicle)
some_vehicle.update(driver: some_char)
client.emit "They are driving: #{some_char.vehicle.name}"

Collections

Some references reflect a one-to-many relationship. A character is in a single room, but a room can hold multiple characters. Collections are the “many” side of the one-to-may relationship. They always must be used in conjunction with a reference.

class Character
   reference :room, "AresMUSH::Room"
end

class Room
   collection :characters, "AresMUSH::Character"
end

in some command handler...
enactor.room.emit "Some message to the room."
enacotr.room.characters.each { |c| ... }  # Do something for each character in the room

Sets

Sets are used when there is a many-to-many relationship between objects. A scene has a set of participants and a participant can be in many scenes.

To define and use a set:

class Scene
  set :participants, "AresMUSH::Character"
end

in some command handler...
scene.participants.add some_char
scene.participants.delete some_char
client.emit "Participants: #{scene.participants.map { |p| p.name }.join(" ")}"