Relationship Examples

# relationships/follow.relationships.causet
relationships:
  artist_followers:
    from: user
    to:   artist
    cardinality: many_to_many
    unique: true
    emit_events:
      created: ARTIST_FOLLOWED
      removed: ARTIST_UNFOLLOWED
 
  show_going:
    from: user
    to:   show
    cardinality: many_to_many
    unique: true
    emit_events:
      created: USER_MARKED_GOING

actions:
  FOLLOW_ARTIST:
    state: user
    entity_id_expr: intent.user_id
    input:
      user_id:   { type: string, required: true }
      artist_id: { type: string, required: true }
    preflight:
      rules:
        - name: not_already_following
          when: { expr: "LOOKUP_IS_FOLLOWING == true" }
          then:
            - op: reject
              code: ALREADY_FOLLOWING
    core:
      rules:
        - name: create_follow
          when: {}
          then:
            - op: relationship_create
              relationship: artist_followers
              from_id: intent.user_id
              to_id:   intent.artist_id

projections:
  user_following:
    source_events: [ARTIST_FOLLOWED, ARTIST_UNFOLLOWED]
    target:
      table: user_following
      primary_key: [user_id, artist_id]
    fields:
      user_id:     TEXT
      artist_id:   TEXT
      followed_at: BIGINT
    derive:
      user_id:     event.user_id
      artist_id:   event.artist_id
      followed_at: event.ts
    mutations:
      ARTIST_FOLLOWED:   { op: upsert }
      ARTIST_UNFOLLOWED: { op: delete }