Working with Collections of Data

Collection resources are common in Liferay DXP web APIs. If you followed along with the previous examples that sent requests to the portal’s blog-postings resource URL, you’ve already seen collections in action: the BlogPosting resource is a collection.

Here, you’ll learn more detailed information about working with collection resources. But first, you should learn about collection pagination.

Pagination

A small collection can be transmitted in a single response without difficulty. Transmitting a large collection all at once, however, can consume too much bandwidth, time, and memory. It can also overwhelm the user with too much data.

It’s therefore best to get and display the elements of a large collection in discrete chunks, or pages.

Liferay DXP’s GraphQL APIs return paginated collections by default. The following attributes in the responses also contain the information needed to navigate between those pages:

totalCount: The total number of this resource’s items.

pageSize: The number of this resource’s items to be included in this response.

page: The current page’s number.

items: The collection elements present on this page. Each element also contains the data of the object it represents, so there’s no need for additional requests for individual elements.

id: Each item’s identifier. You can use this, if necessary, to get more information on a specific item.

The attributes page and pageSize allow client applications to navigate through the results. For example, such a client could send a request for a specific page.

This example gets the second page (page:2) of blog postings that exist on the content set with the ID 42345:

query {
  contentSetContentSetElements(contentSetId: 42345, page: 2) {
    items {
      id
      title
      content {
        ... on BlogPosting {
          headline
        }
      }
    }
    page
    pageSize
    totalCount
  }
}