更新数组：$push、$pull、$pop、$addToSet

MongoDB provides a rich set of array update operators that let you modify arrays inside documents atomically—without reading the document first, modifying it in application code, and writing it back. These operators are far more efficient and safe in concurrent environments than read-modify-write patterns. The main operators are: $push, $pull, $pop, and $addToSet.

$push appends one or more values to the end of an array. If the field does not exist, $push creates it as a new array containing the pushed value. $push allows duplicates—if you push the same value twice, it appears twice in the array. Use it when order and duplicates are acceptable, like append-only log arrays.

To push multiple values at once without multiple updateOne calls, combine $push with the $each modifier. This is more efficient and atomic. You can also combine $each with $position to insert at a specific index, $slice to cap the array length, or $sort to keep the array sorted after the push.

$addToSet adds a value to an array only if it does not already exist. If the value is already in the array, the operation has no effect. This is the correct operator for maintaining a set of unique values, like a list of user tags, role names, or liked post IDs. Unlike $push, $addToSet guarantees no duplicates.

$pull removes all array elements that match a specified condition. Unlike $pop (which removes by position), $pull removes by value or condition. You can pull by exact value, by a query expression, or by a complex condition using $elemMatch-style sub-document matching.

$pull removes all matching elements, not just the first one. If your array has duplicates and you want to remove all copies, $pull handles this in one operation. For removing from multiple documents, combine with updateMany() to pull from every matching document in the collection.

$pop removes the first or last element of an array based on position. Pass 1 to remove the last element (like a stack pop) or -1 to remove the first element (like a queue dequeue). This is useful for maintaining fixed-size sliding windows or queue-like arrays without reading the array first.

$pullAll is a convenience operator that removes all instances of each specified value from an array. It's equivalent to using $pull with $in, but with a slightly simpler syntax when you have a fixed list of values to remove. Note: $pullAll matches exact values and cannot accept query operators.

You can combine multiple update operators in a single updateOne call, but you cannot apply two operators to the same field in one update. You can, for example, simultaneously $push to a tags array and $inc a count field in one atomic operation, reducing round-trips to the database.

The key question when choosing between $push and $addToSet is: do you want to allow duplicates? Use $push when order matters, duplicates are valid, or you're modelling a log/history. Use $addToSet when the array represents a set of unique values like tags, roles, liked IDs, or subscriptions. $addToSet is slightly more expensive because MongoDB checks for existence before inserting.

Arrays that grow without bound cause document bloat and can exceed MongoDB's 16 MB document size limit. Common mitigation strategies include: using $push with $slice to cap the array at a maximum length; periodically running cleanup operations to remove old elements; or using the Bucket Pattern to group entries into time-bounded bucket documents instead of a single unbounded array.

Test your understanding of array update operators in MongoDB.

In this lesson you learned: $push appends elements (with $each, $slice, $sort modifiers), $addToSet adds only unique values, $pull removes all matching elements, and $pop removes the first or last element by position. Next up we explore positional and filtered positional updates for modifying array elements in place.