Relationships

You can model relationships between your data using foreign‑key relationships﻿ and key‑value pair relationships﻿ by way of embedded maps and arrays. For more information about Ditto's advanced types, see Data Types﻿.
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Ditto does not support nesting documents within documents. If you need to create relationships between documents, use a foreign-key relationship by referencing the document ID. For more information, see Foreign-Key Relationships﻿, as follows.
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 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 map 
List items in an array
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 map
Create references between documents in different collections
Foreign-Key Relationship
You can represent a one-to-many or many-to-one relationship between two or more collections, in which one collection refers to the primary key of another collection to create a meaningful relationship between the 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()
})
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Key-Value Relationship
A key-value relationship establishes a parent-child hierarchy between embedded data elements where the key serves as the parent and the associated values for the children to link related data items as well as organize them in a structure that enhances the efficiency of lookup.
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Relationship	Description	Approaches
One-to-many	Associates a parent element with children elements to establish a hierarchy.	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 map List items in an array 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 map Create references between documents in different collections

Updated 11 Mar 2024

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