1. db.cloneCollection()
db.cloneCollection(from, collection, query)
在不同的mongodb实例间复制数据，db.cloneCollection是cloneCollection数据库命令的一个外在体现。

function (from, collection, query) {
 assert( isString(from) && from.length );
 assert( isString(collection) && collection.length );
 collection = this._name + "." + collection;
 query = query || {};
 return this._dbCommand( { cloneCollection:collection, from:from, query:query
} );
}