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([{$addFields:{category:{$map:{input:{$split:["$category",","]},in:{$toInt:{$trim:{input:"$$this"}}}}}}},{$lookup:{from:"categories",as:"category",localField:"category",foreignField:"_id"}},{$unwind:{path:"$category",preserveNullAndEmptyArrays:true}},{$project:{_id:"$category._id",categoriesText:"$category.text"}}])