There are several methods for linking related data items and organizing them for easy lookup:

  • Field referencing another document by _id.
  • Embedded JSON object that acts as a REGISTER type or an embedded MAP.

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Overview

The following table provides a complete overview of the different relationships you can form in Ditto, as well as a brief description, list of possible approaches you can take, and links to related content:

RelationshipDescriptionApproaches
One-to-manyAssociates a parent element with children elements to establish a hierarchy.* Embed a JSON object (REGISTER)

* Embed a MAP

* Reference a field to a document

* Reference a document to a collection
Many-to-manyAssociates multiple entities in one collection with multiple entities in another collection.* Embed a JSON object (REGISTER)

* Embed a MAP

* Create references between documents in different collections
Many-to-oneAssociates two or more collections, where one collection refers to the primary key of another collection to create a meaningful relationship between the datasets.* Embed a JSON object (REGISTER)

* Embed a MAP

* Create references between documents in different collections

Avoid using arrays in Ditto.

Due to potential merge conflicts when offline peers reconnect to the mesh and attempt to sync their updates, especially when multiple peers make concurrent updates to the same item within the array.

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Foreign-Key Relationships

To create a foreign-key relationship, store the primary key to one document within another document.

A foreign-key relationship establishes a link between two or more datasets. For example, if you have two collections, cars and owners, where each car has a corresponding owner, every document in the cars collection would include a field containing the ID of a document in the owners collection.

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Key-Value Relationships

A *key-value relationship *establishes a parent-child hierarchy between embedded data elements. In this hierarchy, the key functions as the parent, and its encapsulated values, represented as a set of key-value pairs, serve as children.

When managing data that requires unique identifiers and relationships, instead of using an array to encode your data, use a MAP with unique string keys and object values instead.

If you need to represent a highly complex dataset in a document, you can embed a MAP data type within a document.

Avoid using arrays in Ditto.

Due to potential merge conflicts when offline peers reconnect to the mesh and attempt to sync their updates, especially when multiple peers make concurrent updates to the same item within the array.

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Deeply-Hierarchical Structures

Embedding a MAP is one way to structure and organize related data within a single document to create a complex structure with multiple levels of hierarchy. As in, you can embed a MAP within a MAP, within another MAP, and so on.

For example, the following snippet shows three levels of embedded maps: details, engine, interior, and features.

JSON
{
  "_id": {
    "vin": "123abc",
    "make": "Toyota",
    "model": "Corolla",
    "year": 2022,
  },
  "details": {
    "engine": {
      "type": "Gasoline",
      "displacement": "1.8L"
    },
    "interior": {
      "seats": 5,
      "color": "Black"
    },
    "features": {
      "safety": {
        "airbags": 6,
        "antilockBrakes": true
      },
      "technology": {
        "infotainment": "Touchscreen",
        "navigation": true
      }
    }
  }
}

Each level contains its key-value pairs and, if used, children-level MAP. You can represent key values using a REGISTER, ATTACHMENT, or another MAP. 

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Benefits of Embedding Maps

Embedding a MAP is beneficial in scenarios where you need to manage a collection of items and continuously modify that collection over time; that is you want to link multiple data items with a single unique string identifier, but you anticipate that these data items are subject to concurrent edits over time.

For example, consider a basic Point-of-Sale (PoS) system where you need to keep track of customer orders. Since multiple users can add and remove orders within the collection, you would embed a map to represent the ordered items. In this map, each key denotes an item ID, and the corresponding value indicates the quantity ordered.

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