# Child Collections

A child collection is a container for data models that are tied to a parent collection. The child collection system shares the [Collections](https://docs.umbraco.com/umbraco-ui-builder/collections/the-basics) API, offering flexibility for managing and displaying related data within your backoffice UI.

![Child Collections](https://2099605805-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FXdAOUcqxoBqXe5oCQF8z%2Fuploads%2Fgit-blob-b0f844ceeb89fa4303cebdf79670a31c5921c4ca%2Fchild_collections.png?alt=media)

By default, child collections are displayed as context apps within the parent model's editor view. If multiple child collections lead to an overcrowded context apps area, consider using the [Child Collection Groups API](https://docs.umbraco.com/umbraco-ui-builder/collections/child-collections/child-collection-groups). Using the API, you can group related child collections under a single context app, with each child collection appearing in separate tabs.

## Defining a Child Collection

To define a child collection, use the `AddChildCollection` method on the given collection config builder instance.

### Using the `AddChildCollection()` Method

This method adds a child collection with the specified names, description, and default icons. Both the entity ID and foreign key fields must be specified using property accessor expressions.

#### Method Syntax

```cs
AddChildCollection<TChildEntityType>(Lambda idFieldExpression, Lambda fkFieldExpression, string nameSingular, string namePlural, string description, Lambda childCollectionConfig = null) : ChildCollectionConfigBuilder<TEntityType>
```

#### Example

```csharp
collectionConfig.AddChildCollection<Child>(c => c.Id, c => c.ParentId, "Child", "Children", "A collection of children", childCollectionConfig => {
    ...
});
```

### Using the `AddChildCollection()` Method with Custom Icons

This method adds a child collection to the current collection with the specified names, description and custom icons. Both the entity ID and foreign key fields must be specified using property accessor expressions.

#### Method Syntax

```cs
AddChildCollection<TChildEntityType>(Lambda idFieldExpression, Lambda fkFieldExpression, string nameSingular, string namePlural, string description, string iconSingular, string iconPlural, Lambda childCollectionConfig = null) : ChildCollectionConfigBuilder<TEntityType>
```

#### Example

```csharp
collectionConfig.AddChildCollection<Child>(c => c.Id, c => c.ParentId, "Child", "Children", "A collection of children", "icon-umb-users", "icon-umb-users", childCollectionConfig => {
    ...
});
```

## Configuring a Child Collection

Child collections share the same API as the `Collection` config builder API, except child collections cannot contain further child collections. For more information, see the [Basics](https://docs.umbraco.com/umbraco-ui-builder/collections/the-basics) article.