Relationships

There are several methods for linking related data items and organizing them for easy lookup:
Field referencing another document by _id. (See Foreign-Key Relationships﻿) 
Embedded JSON object that acts as a REGISTER type or an embedded MAP. (See Key-Value Relationships﻿)
For an example demonstrating both the deeply embedded and flat models, see Complex Structures﻿.
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:
Relationship
Description
Approaches
One-to-many
Associates 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-many
Associates 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-one
Associates 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
Foreign-Key Relationships
To create a foreign-key relationship, store the primary key to one document in another document. 
A foreign-key relationship establishes a link between two or more datasets. For example, the following snippet demonstrates a foreign-key relationship between documents in the cars and people collections, in which the reference to susanId serves as the foreign key establishing a relationship between cars and people:
pseudocode
const results = await ditto.store.write(async (transaction) => {
   // Create a person named Susan in the "people" collection
  const cars = transaction.scoped('cars')
  const people = transaction.scoped('people')

  // Create a car document in the "cars" collection
  const susanId = await people.upsert({
    name: 'Susan',
  })
  await cars.upsert({
    make: 'Hyundai',
    color: 'red',
    owner: susanId, // Set the owner field to Susan's ID
  })
  
  // Evict the Susan document from the "people" collection
  await people.findByID(susanId).evict()
})
const results = await ditto.store.write(async (transaction) => {
   // Create a person named Susan in the "people" collection
  const cars = transaction.scoped('cars')
  const people = transaction.scoped('people')

  // Create a car document in the "cars" collection
  const susanId = await people.upsert({
    name: 'Susan',
  })
  await cars.upsert({
    make: 'Hyundai',
    color: 'red',
    owner: susanId, // Set the owner field to Susan's ID
  })
  
  // Evict the Susan document from the "people" collection
  await people.findByID(susanId).evict()
})
﻿
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. 
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Relationship	Description	Approaches
One-to-many	Associates 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-many	Associates 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-one	Associates 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

Updated 12 Mar 2024

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