Скачать или смотреть How to Merge MongoDB Arrays into One Array with Aggregation

Learn how to effectively merge arrays in MongoDB using aggregation framework step-by-step to achieve your desired results.
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Merging MongoDB Arrays with Aggregation: A Step-by-Step Guide

When working with MongoDB, you might often find yourself needing to merge arrays from multiple documents into a single array for easier manipulation and visualization. In this guide, we're going to tackle a common problem: merging arrays containing tab values associated with the same name from a collection of documents.

The Problem at Hand

Suppose you have a MongoDB collection structured as follows:

[[See Video to Reveal this Text or Code Snippet]]

Your goal is to transform the data into a single document that looks like this:

[[See Video to Reveal this Text or Code Snippet]]

But when trying to achieve this, you might run into issues with your initial aggregation query:

[[See Video to Reveal this Text or Code Snippet]]

This query does not produce the desired output because the $push operator collects arrays as arrays instead of flattening them into a single array.

The Solution: Using $unwind and $group

To successfully merge the arrays, we can utilize the $unwind and $group stages in the aggregation pipeline. Here’s how you can construct the correct query:

Step-by-step Breakdown

Unwind the tab array:

The first step is to unwind the tab array. This will create a separate document for each element in the array, allowing us to handle each value individually.

Group the results:

After unwinding, the next step is to group the documents by name. In this stage, we will use $push again, but now it will correctly collect individual tab values into a single merged array.

Project the final structure:

The last step is to project the fields we need, making sure to format them appropriately.

The Final Query

This is the final aggregation query that will achieve our desired output:

[[See Video to Reveal this Text or Code Snippet]]

Explanation of the Query

$unwind: "$tab":

This breaks down the array into individual documents, each with a single tab value.

$group:

_id: "$name" groups all documents by the name, while mergedTab: { $push: "$tab" } collects all the unwound tab values.

$project:

This re-shapes the output to match your expected structure, removing the _id field of the group.

Conclusion

By following the steps outlined in this guide, you can easily merge arrays in MongoDB using the aggregation framework. This approach will help you manipulate your data more effectively and present it in your desired format.

Feel free to try out the provided solution in your MongoDB environment and tweak it based on your specific needs!