code: function buildLeftJoinRelations () {
      // if no left joins specified do nothing
      if (!this.args.left || !this.args.left.join) {
          return
      }
      // get join as array
      var joins = Array.isArray(this.args.left.join) ? this.args.left.join : [this.args.left.join]
      // add each join
      _.each(joins, join => {
          // join may be passed as string that is name of relation to join on
          if (typeof join === 'string') {
              join = {
                  relation: join,
              }
          }
          // require join to be object
          assert.ok(join && typeof join === 'object', 'left join must be string or object')
          // set the left join flag to true
          join.left = true
          // add join to query
          this.buildJoinRelation(join)
      })
  }
docs: @function buildLeftJoinRelations
  
  build left joins on relations
  
  @throws {Error}