# Relationships

If certain collections within your project are related then you can connect them with Relationships. There are multiple types of relationships but technically Directus only needs to store one: the Many-to-One.

# Many-to-One

A many-to-one (M2O) relationship exists when an item of Collection A is linked to one single item of Collection B, but an item of Collection B may be linked to many items of Collection A. For example, a movie has one director, but directors have many movies.

# Setup

This setup is specific to the movies → directors (M2O) field. The following steps assume you already have two collections: movies and directors. Each collection has the default id primary key and a name field.

# Screenshots

Both dropdowns under "This Collection" are disabled since those refer to the field we're configuring now. The Related Field is also disabled since it must be the collection's Primary Key. All you need to do is choose which collection you want to relate to... in this case: directors.

# One-to-Many

A one-to-many (O2M) relationship exists when an item of Collection A may be linked to many items of Collection B, but an item of Collection B is linked to only one single item of Collection A. For example, directors have many movies, but a movie only has one director. As you can see, this is the same relationship as the M2O above... but looking at it from the opposite direction.

# Setup

This setup is specific to the directors → movies (O2M) field. The following steps assume you already have two collections: movies and directors. Each collection has the default id primary key and a name field. Additionally, we're assuming you have already created the M2O relationship above, which creates the movies.director field.

# Screenshots

Both dropdowns under "This Collection" are disabled since those refer to the field we're configuring now. First, choose the Related Collection, in this case movies. Once that is selected the Field dropdown will update to show the allowed options and you can choose the field that will store the foreign key in the related collection. In this example, movies.director will store director.id so we choose director.

# Direction Matters

Now we understand that a M2O and O2M are the exact same relationship... just viewed from opposite directions. The movies form shows a M2O dropdown to choose the director, and the directors form has a O2M listing to select their movies. But if you were to peek behind the scenes you would only see one entry in directus_relations defining this duplex relationship.

# One-to-One

Directus does not have a specific one-to-one (O2O) relationship type or interface. However a O2O saves data the same way as a M2O (storing a foreign key) — the only difference is how you enforce the cardinality. By default, the M2O allows you to create multiple relationships in one direction, but it can become a O2O by adding an event hook (item.create:before and item.update:before) or a custom interface that checks/constrains uniqueness.

# Many-to-Many

A many-to-many (M2M) is a slightly more advanced relationship that allows you to link any items within Collection A and Collection B. For example, movies can have many genres, and genres can have many movies.

Technically this is not a new relationship type, it is a O2M and M2O working together across a "junction" collection. Each item in the junction (eg: movie_genres) is a single link between one item in movies and one item in genres.

# Setup

This setup is specific to the movies → genres (M2M) field. The following steps assume you already have two collections: movies and genres. Each collection has the default id primary key and a name field.

# Screenshots

Both dropdowns under "This Collection" are disabled since those refer to the field we're configuring now. The Related Field is also disabled since it must be the collection's Primary Key. First, choose the collection you want to relate to. Now select a junction collection and connect its keys by following the arrows.

# Many-to-Any

The many-to-any (M2X) allows you to connect items within Collection A to many items from any collection. It is essentially the same as a M2M, but requires connected collections to use a Universally Unique Identifier (UUID) for the primary key. The Directus relational architecture supports this type of relationship, but there is no dedicated M2X interface yet.

This type of relationship goes by many different names, and is often referred to by its specific purpose. Some names include: matrix field, replicator, M2MM, M2X, M2N, etc.

# Translations

The translation interface is a standard O2M relation, but it requires an additional field in the related collection to hold the language key. Let's take a look at some example collections and fields for "Articles":

• articles — The parent collection
  • id — The article's primary key
  • publish_date — Example agnostic field
  • translations — This is the ALIAS field (not an actual column) we're setting-up
• article_translations — The translations collection
  • id — The translation's primary key
  • article — Stores the article's primary key: article.id
  • language — Stores the language key, eg: en-US
  • title — Example translated field
  • body — Example translated field

# The Parent Collection

In this example, the parent collection stores articles, so we've called it articles. It contains a primary key, as well as any language-agnostic (non-translated) fields, such as: Publish Date, Author, or a Featured Toggle. We also add the "translation field" (actually an ALIAS) to this collection... which is what we're learning to set up here.

# The Translations Collection

We also need to create a related collection to store any fields that will be translated, such as the Title and Body. We'll call this article_translations. Below we describe the required fields for this collection.

• Parent Foreign Key — This is the field that stores the parent item's primary key. So in our example we would add an article field to store the article's ID. Typically we'd use a "Primary Key" interface, and set it to "Hide on Detail".
• Language Foreign Key — This is the field that stores the language code. We recommend calling this field language. Typically we'd use a "Language" interface with a lang type, and set it to "Hide on Detail". We also recommend setting the option "Limit to Directus Availability".
• One or More Translated Fields — Now just add any fields that you would like to use for translated content. In our "Articles" example this might be a title field as a Text Input, a body field as a WYSIWYG, or anything else you want translated.

# The Translation Field Setup

Once you've set up both the parent and translation collections (eg: articles and article_translations), you can follow these steps to setup the translations interface.