Relationships
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.
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. |
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Many-to-many | Associates multiple entities in one collection with multiple entities in another collection. |
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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. |
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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, 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:
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.
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.
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 an example demonstrating both the deeply embedded and flat models, see Bad Pattern: Large Documents.
For example, the following snippet shows three levels of embedded maps: details, engine, interior, and features.
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.
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.
As an example, the following snippet demonstrates a basic Point-of-Sale (PoS) system where you need to keep track of the customer orders collection. And, since multiple users can add and remove orders within the collection, you embed a map to represent the ordered items, where each key denotes an item ID and the linked value indicates the quantity ordered: