Using MongoDB Aggregation to Build Hierarchical Category Structures

In application development, it’s often necessary to manage and display hierarchical category structures, such as parent categories, subcategories, and sub-subcategories. MongoDB provides a very useful tool for this task through the aggregation framework, specifically with the $graphLookup stage. Will explain how you can use MongoDB Aggregation Framework to build hierarchical category structures from data stored in a MongoDB collection.

1. Data Structure

First, let’s look at an example of category data stored in a MongoDB collection:

{ "_id" : 1, "cat_id" : 1, "title" : "Parent Category", "parent" : null }
{ "_id" : 2, "cat_id" : 2, "title" : "Child Category", "parent" : 1 }
{ "_id" : 3, "cat_id" : 3, "title" : "Sub Child Category", "parent" : 2 }

In this example:

• Parent Category is the parent category.
• Child Category is a subcategory of Parent Category.
• Sub Child Category is a sub-subcategory of Child Category.

2. Using $graphLookup to Build Category Structure

MongoDB provides $graphLookup in the aggregation pipeline to perform recursive lookups within the collection. This is very useful for building hierarchies like category structures.

Basic syntax for $graphLookup:

db.categories.aggregate([
  {
    $match: { "parent": null }
  },
  {
    $graphLookup: {
      from: "categories",
      startWith: "$_id",
      connectFromField: "_id",
      connectToField: "parent",
      as: "subCategory"
    }
  }
])

Explanation:

• $match: { "parent": null }: Filters documents to get the parent categories (categories that do not have a parent).
• $graphLookup: Performs a recursive search to find all related subcategories.
  • from: The name of the collection used for the lookup (in this case, the same collection categories).
  • startWith: The field from the initial document used as the starting point for the lookup (here, _id of the parent category).
  • connectFromField: The field in the initial document used for the lookup (i.e., _id).
  • connectToField: The field in the documents being looked up that will be connected (i.e., parent).
  • as: The name of the new field that will contain the recursive lookup results (in this case, subCategory).

3. Aggregation Results

The result of this aggregation operation will provide a hierarchical category structure with all related subcategories. Here’s the result from the example data provided:

{ 
  _id: 1,
  cat_id: 1,
  title: 'Parent Category',
  parent: null,
  subCategory: [
    { _id: 2, cat_id: 2, title: 'Child Category', parent: 1 },
    { _id: 3, cat_id: 3, title: 'Sub Child Category', parent: 2 }
  ]
}

In this result, Parent Category has a subCategory field containing all related subcategories, including Child Category and Sub Child Category.

4. Conclusion

The MongoDB Aggregation Framework with $graphLookup allows you to efficiently build and display hierarchical category structures. By using this approach, you can easily manage hierarchical data in your application, providing a robust way to organize and display structured information. This technique is valuable in many applications, including e-commerce, content management systems, and more.