db={"hero_foods":[{"_id":"Almond Butter","title":"Almond Butter","food_group":"protein","recommendations_group":["DT1","DT2","DT3","DT4"]}],"recommendation_items":[{"_id":"DT1","type":"DIET","title":"pfc","recommendation_items":{"title":"What you'll eat","food_groups":[{"title":"Quality Carbs","category_id":"carbs"},{"title":"Protein","category_id":"protein"},{"title":"Healthy Fats","category_id":"fats"}]}}]}

db.recommendation_items.aggregate([{$match:{type:"DIET"}},{$lookup:{from:"hero_foods",localField:"_id",foreignField:"recommendations_group",as:"recommendation_items.hero_foods_list"}},{$set:{"recommendation_items.food_groups":{$map:{input:"$recommendation_items.food_groups",as:"foodGroup",in:{$mergeObjects:["$$foodGroup",{hero_foods_list:{$filter:{input:"$recommendation_items.hero_foods_list",as:"heroFood",cond:{$eq:["$$foodGroup.category_id","$$heroFood.food_group"]}}}}]}}}}}])