find all records that have empty / no associations (left_outer_joins)
Rails
AREL trick
let say we have M:M relation
- maps
- maps_strategies
- maps_strategy_maps (join table)
and you want to select items that are not joined in that other table (Maps that are not assigned to any MapStrategy)
@maps = Map.all
# just Maps with no strategy
@maps_without_strategy = @maps.left_outer_joins(:map_strategies).where(map_strategies: {id: nil})Or you can do
@maps_without_strategy = @maps.where.not(id: MapStrategyMap.select("map_id"))