Database

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Query

db.collection.aggregate([{"$match":{"_id":ObjectId("609b2ab4c60588f6b0579259")}},{$addFields:{columns:{$map:{input:"$columns",in:{$mergeObjects:["$$this",{cards:{$map:{input:"$$this.cards",in:{$mergeObjects:["$$this",{attachments:{$size:{$ifNull:["$$this.attachments",[]]}}},{checkLists:{$size:{$ifNull:["$$this.checkLists",[]]}}},{activity:{$size:{$ifNull:["$$this.activity",[]]}}}]}}}}]}}}}}])

Result