$push accumulator for building arrays in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of this aggregation using $push?

Consider a collection orders with documents:

{"_id": 1, "item": "apple", "qty": 5}
{"_id": 2, "item": "banana", "qty": 10}
{"_id": 3, "item": "apple", "qty": 15}

What will be the result of this aggregation?

db.orders.aggregate([
  { $group: {
      _id: "$item",
      quantities: { $push: "$qty" }
  }}
])

MongoDB

db.orders.aggregate([
  { $group: {
      _id: "$item",
      quantities: { $push: "$qty" }
  }}
])

A [ { _id: "apple", quantities: [5, 15] }, { _id: "banana", quantities: [10] } ]

B [ { _id: "apple", quantities: [5] }, { _id: "banana", quantities: [10, 15] } ]

C [ { _id: "apple", quantities: 20 }, { _id: "banana", quantities: 10 } ]

D [ { _id: "apple", quantities: [15, 5] }, { _id: "banana", quantities: [10] } ]

💡 Hint

The $push accumulator collects values from each grouped document into an array in the order they appear.

The $group stage groups documents by the 'item' field. For each group, $push collects the 'qty' values into an array preserving order. For 'apple', qty values 5 and 15 are pushed in order. For 'banana', only qty 10 exists.

🧠 Conceptual

intermediate

What does the $push accumulator do in MongoDB aggregation?

Choose the best description of the $push accumulator in MongoDB aggregation framework.

A It returns the first value of a field in each group.

B It removes duplicate values from an array.

C It sums numeric values for each group.

D It adds values to an array for each group, preserving the order of documents.

💡 Hint

Think about how $push collects values during grouping.

$push collects values from each document in a group into an array, preserving the order they appear in the input documents.

📝 Syntax

advanced

Which aggregation pipeline stage correctly uses $push to collect tags?

Given a collection posts with documents containing a tags array, which pipeline stage correctly groups by author and pushes all tags into one array?

A { $group: { _id: "$author", allTags: { $push: "$tags" } } }

B { $group: { _id: "$author", allTags: { $push: "$tags" }, $unwind: "$tags" } }

C { $group: { _id: "$author", allTags: { $push: { $unwind: "$tags" } } } }

D { $group: { _id: "$author", allTags: { $push: "$tags" }, $addToSet: "$tags" } }

💡 Hint

Remember $push collects values as they are; $unwind is a separate stage.

Option A correctly groups by author and pushes the entire tags array from each document into an array of arrays. Options A, C, and D misuse $unwind or $addToSet inside $group stage syntax.

🔧 Debug

advanced

Why does this aggregation pipeline fail?

Given this pipeline:

db.sales.aggregate([
  { $group: {
      _id: "$store",
      items: { $push: "$item" },
      total: { $sum: "$amount" }
  } },
  { $project: { _id: 0, store: "$_id", items: 1, total: 1 } }
])

The query returns an error. What is the cause?

A The $project stage tries to rename '_id' to 'store' but must use $set instead.

B The $project stage uses an invalid field name 'store' referencing '$_id' incorrectly.

C The $group stage uses $push and $sum together which is not allowed.

D The $group stage is missing a closing brace.

💡 Hint

Check how fields are referenced in $project stage.

In $project, to rename _id to store, you must use store: "$_id". This syntax is correct, so the error is likely elsewhere. But here, the error is a common confusion: MongoDB expects field names without quotes in $project. Using quotes around '$_id' causes an error.

❓ optimization

expert

How to optimize $push usage to avoid nested arrays when pushing array fields?

You have documents with a field tags which is an array. You want to group by category and collect all tags into a single flat array (not array of arrays). Which approach is best?

A Use $push: "$tags" in $group and accept nested arrays; flatten in application code.

B Use $addToSet: "$tags" in $group to avoid duplicates and flatten arrays automatically.

C Use $push: "$tags" in $group, then use $reduce or $concatArrays in a $project stage to flatten.

D Use $push: "$tags" in $group, then $unwind the resulting array after grouping.

💡 Hint

Think about how to flatten arrays inside aggregation pipeline.

Using $push on an array field creates an array of arrays. To flatten, you can use $reduce with $concatArrays in a $project stage after grouping. $addToSet does not flatten arrays automatically. $unwind after grouping is inefficient and changes document count.