事先占位然后删除

预先占位然后删除

预先占位在一个额外的字段，然后取消

comments = [ {name : "Name Surname", date : "1970.01.01", text : "this is a placeholder"},             {name : "Name Surname", date : "1970.01.01", text : "this is a placeholder"} ]db.posts.insert({  comments: [],  padding: 'lorep ipsum ... x N'})db.posts.update(query,                 {$push: {comments: comment_data},                 $unset: padding})

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