db={categories:[{"_id":231,"text":"category 231",},{"_id":245,"text":"category 245",},{"_id":276,"text":"category 276",}],posts:[{"_id":71,category:"245, 276"}]}

db.posts.aggregate([{$lookup:{from:"posts",let:{postId:71},as:"post",pipeline:[{$match:{$expr:{$eq:["$$postId","$_id"],}}},{$lookup:{from:"categories",as:"category",let:{category:{$toInt:{$arrayElemAt:[{$split:["$category",","]},0]}},},pipeline:[{$match:{$expr:{$eq:["$$category","$_id"]}},},]}},{$unwind:{path:"$category",preserveNullAndEmptyArrays:true}},{$addFields:{categoriesText:"$category.text",}},{"$project":{"category":0,}}]},},])