Code Summary: Inheritance Pattern

To apply the Inheritance Pattern to documents in a collection, you can use the aggregation framework.

This first example updates all book documents with a new “product type” field to identify the shape of the ebook, printed book, and audiobook documents. The pipeline also adds a product_id field for all documents and makes shared attributes more consistent. It does this by changing desc and details fields to description and by converting the author field to an array. Finally, it replaces the existing documents in the same collection.

var apply_inheritance_pattern_to_books_pipeline = [
  {
    $project: {
      _id: "$_id",
      product_id: "$product_id",
      product_type: {
        $ifNull: ["$product_type", "Unspecified"],
      },
      description: {
        $ifNull: [
          "$desc",
          "$description",
          "$details",
          "Unspecified",
        ],
      },
      authors: {
        $ifNull: [
          "$authors",
          ["$author"],
          "Unspecified",
        ],
      },
      publisher: "$publisher",
      language: "$language",
      pages: "$pages",
      catalogues: "$catalogues",
      eformats: "$eformats",
      isbn10: "$isbn10",
      isbn13: "$isbn13",
      narrator: "$narrator",
      length_minutes: "$length_minutes",
    },
  },
  {
    $merge: {
      into: "books",
      on: "_id",
      whenMatched: "replace",
      whenNotMatched: "discard",
    },
  },
]

db.books.aggregate(apply_inheritance_pattern_to_books_pipeline)

The next pipeline changes all documents with a product type of “unspecified” with a minute length greater than zero to “audiobook” and saves them to the collection.

var cleanup_audiobook_entries_in_book_pipeline = [
  {
    $match: {
      $and: [{ product_type: "Unspecified" }, { length_minutes: { $gte: 0 } }],
    },
  },
  {
    $set: { product_type: "audiobook" },
  },
  {
    $merge: {
      into: "books",
      on: "_id",
      whenMatched: "replace",
      whenNotMatched: "discard",
    },
  },
];

db.books.aggregate(cleanup_audiobook_entries_in_book_pipeline);

Finally, we ran the following command to find one of the updated documents in order to view the changes.

db.books.find({ _id: 3 })
[
  {
    _id: 3,
    product_id: 54538756,
    product_type: 'audiobook',
    description: 'The complete book of MongoDB by its employees',
    authors: [ 'Eoin Brazil' ],
    publisher: "O'Reilly",
    language: 'English',
    narrator: 'Eoin Brazil',
    length_minutes: 1200
  }
]