Mongodb aggregation lookup with conditions

Question

I have a collection called article_category which store all article_id belongs to the category with category_id with data format like

Answer 1

You've couple of things incorrect here. category_id should be all_category_id. Use the join condition in $lookup and move the $match outside of $lookup stage with $unwind for optimized lookup.

Use $project with exclusion to drop the looked up field from final response. Something like {$project:{article_category:0}}

Try

db.article.aggregate([
  {"$match":{"title":{"$regex":/example/}}},
  {"$lookup":{
    "from":"article_category",
    "localField":"article_id",
    "foreignField":"article_id",
    "as":"article_category"
  }},
  {"$unwind":"$article_category"},
  {"$match":{"article_category.all_category_id":8}}
])

For uncorrelated subquery try

db.article.aggregate([
  {"$match":{"title":{"$regex":/example/}}},
  {"$lookup":{
    "from":"article_category",
    "pipeline":[{"$match":{"all_category_id":8}}],
    "as":"categories"
  }},
  {"$match":{"categories":{"$ne":[]}}}
])