[{user_id:"user1",books:[{id:"book1",title:"this is book1"},{id:"book1",title:"this is book1"}]},{user_id:"user2"}]

db.collection.update({},[{$set:{books:{$concatArrays:[{$ifNull:["$books",[]]},[{id:"book1",title:"this is book1"}]]}}}],{multi:true})