Links are a powerful way to model relationships between content. You can use a URI query parameter with the Contentful search to retrieve an entire chain of related content to display in your application.

Entries can have link fields which point to other entries or assets, for example:

• A restaurant linking to its menu (singular relationship).
• A menu linking to its specific menu items (plural relationship).
• Each menu item linking to a photo (attachment).
• A restaurant linking to multiple photos (attachments).

A single HTTP request lets you retrieve the entire set of linked resources above, starting with the menu, in one request. Contentful's CDN can cache these requests to further speed up future requests. This is useful for mobile apps as it reduces the need for multiple concurrent connections, and the latency for results to return.

Links bring you other features:

• Relationships are clearly defined and validated by specific content type fields.
• Entry links can be validated by content type. E.g. Only allow Menu Items for fields.menuItems.
• Asset links can be validated by file type. E.g. Only allow Images for fields.photo.

Link level

By default, a response includes the first level of linked content. Use the include parameter to set the number of levels you want to return. The maximum number of inclusions is 10.

Link resolution works regardless of how many results are there in items.

JSON response

In the JSON response of a successful query, linked items are placed in the includes array, when not already fetched in the items array.

Take this query:

curl -v 'https://cdn.contentful.com/spaces/oc3u7dt7mty5/entries?access_token=6cabb22c95d52aa7752fe70ae9b3271a1fc2decf7ae7d99ccd7dceba718980e6&include=1'

This fetches the restaurant entry Spaceburger alongside its image links by using the restaurantImages linking field:

"items": [
  {
    "sys": {
      "type": "Entry",
      "id": "2UmoQ8Bo4g4S82WmGiQIQE",
      ...
    },
    "fields": {
      "name": "Spaceburger",
      "description": "The Jetson's love to come here!",
      "restaurantImages": [
        {
          "sys": {
            "type": "Link",
            "linkType": "Asset",
            "id": "23qqdlTciMGm6IYy224euu"
          }
        }
      ]
    }
  },

  ...
],

The response retrieves the image with id=23qqdlTciMGm6IYy224euu as a link to an asset. Information about the 23qqdlTciMGm6IYy224euu image is in the includes array:

"includes": {
  "Asset": [
    {
      "sys": {
        "type": "Asset",
        "id": "23qqdlTciMGm6IYy224euu",
        ...
      },
      "fields": {
        "title": "The SpaceBurger Photo",
        "file": {
          "fileName": "spaceburger.jpg",
          "contentType": "image/jpeg",
          "details": {
            "image": {
              "width": 550,
              "height": 421
            },
            "size": 205683
          },
          "url": "//images.ctfassets.net/oc3u7dt7mty5/23qqdlTciMGm6IYy224euu/85514b430c28045a3b2930ebe15dfcce/spaceburger.jpg"
        }
      }
    },
    ...
  ]
}

Before resolving links to items, Contentful matches the filter conditions of a query. This changes the response contained within the items array to reflect the search criteria of the querying URL.

Here's the response for a menu linked to its meals:

"items": [
    {
      "sys": {
        "type": "Entry",
        "id": "4rJn9OJsBiAKmeoiiw40Ko",
      },
      "fields": {
        "name": "Menu for Humans",
        ...
        "stickiness": 999.3,
        "menuMeal": [
          {
            "sys": {
              "type": "Link",
              "linkType": "Entry",
              "id": "3HkMtbj6hqcMYEqWIOm6SQ"
            }
          },
          {
            "sys": {
              "type": "Link",
              "linkType": "Entry",
              "id": "3SVh6Ei9pCkkIkoE0ME4Ms"
            }
          }
        ]
      }
    },
    ...
    {
      "sys": {
        "type": "Entry",
        "id": "3HkMtbj6hqcMYEqWIOm6SQ",
        ...
      },
      "fields": {
        "weight": 203.1,
        "name": "AstroChicken ",
        "rating": 100,
        "description": "An entire chicken with Andromeda's sauce"
      }
    },
    {
      "sys": {
        "type": "Entry",
        "id": "3SVh6Ei9pCkkIkoE0ME4Ms",
        ...
      },
      "fields": {
        "weight": 23049950.9,
        "name": "AstroCattle",
        "rating": 30,
        "description": "Yummy Cows with fries and Ketchup"
      }
    },
    ...

]

As AstroChicken and AstroCattle all match the conditions of the query parameters and are linked to Menu for Humans, they are all fetched in the same items array. Since AstroChicken and AstroCattle are present in the response's items, they are not included in the includes.Entry array.

Fetching resources linked to a specific entry

You might want retrieve all items linked to a particular target entry. Your query URL should filter entries based on their specific content_type, a linking_field to link items and a entry_id from the target entry.

For example, to retrieve all resources of content type Menu linked to the restaurant Space Burger by using the following query URL:

curl -v 'https://cdn.contentful.com/spaces/oc3u7dt7mty5/entries?access_token=6cabb22c95d52aa7752fe70ae9b3271a1fc2decf7ae7d99ccd7dceba718980e6&content_type=3HjHXUYR3yyosUqAGmi8wu&fields.restaurantField.sys.id=2UmoQ8Bo4g4S82WmGiQIQE'

Because these are all Menus linked to the Space Burger restaurant, Menu for Humans, Menu for Romulans and Menu for Klingons are all retrieved alongside their own links and the target entry:

  "items": [
    {
      "fields": {
        "restaurantField": {
            "sys": {
              "type": "Link",
              "linkType": "Entry",
              "id": "2UmoQ8Bo4g4S82WmGiQIQE"
            }
          },
        "name": "Menu for Humans",
        "images": [
          ...
        ],
        "menuMeal": [
          ...
        ]
      },
      "sys": {
        "type": "Entry",
        "id": "4rJn9OJsBiAKmeoiiw40Ko",
      }
    },
    {
      "fields": {
        "restaurantField": {
          "sys": {
            "type": "Link",
            "linkType": "Entry",
            "id": "2UmoQ8Bo4g4S82WmGiQIQE"
          }
        },
        "name": "Menu for Romulans",
        "menuMeal": [
          ...
        ]
      },
      "sys": {
        "type": "Entry",
        "id": "2Mt2YctJQ4am8u2oI4kcsS",
        ...
      }
    },
    {
      "fields": {
        "restaurantField": {
          "sys": {
            "type": "Link",
            "linkType": "Entry",
            "id": "2UmoQ8Bo4g4S82WmGiQIQE"
          }
        },
        "name": "Menu for Klingons",
        "menuMeal": [
          {
            ...
            }
        ]
      },
      "sys": {
        "type": "Entry",
        "id": "2RnAOt0ssgQ6kIk0E4WAeq",
        ...
      }
    },
    {
      "sys": {
        "type": "Entry",
        "id": "2UmoQ8Bo4g4S82WmGiQIQE",
        ...
      },
      "fields": {
        "name": "Spaceburger",
        "description": "The Jetson's love to come here!",
        "restaurantImages": [
          {
            "sys": {
              "type": "Link",
              "linkType": "Asset",
              "id": "23qqdlTciMGm6IYy224euu"
            }
          }
        ]
      }
    },
    ...
  ]

Modeling Relationships

Linking an entry to another entry represents a relationship. The structure of linked items should be like this:

{
  "fields": {
      "reference_field": {
          "en-US": {
              "sys": {
                  "type": "Link",
                  "linkType": "Entry",
                  "id": "<ID_of_Linked_Item>"
               }
           }
       },
      "title": {
        ...
       },
       ...
   }
}

Here's a restaurant linked to its menu:

{
  "sys": {
    "type": "Entry",
    "id": "il-doges"
  },
  "fields": {
    "menu": {
      "en-US": {
        "sys": {
          "type": "Link",
          "linkType": "Entry",
          "id": "il-doges-nice-menu"
        }
      }
    }
  }
}

Multiple links

It's possible to create circular links, for example a chain of entries to model a dialog in a video game or complex graphs.

Certain fields in every Resource's sys object are also links. These include the space they're in, their content type (in the case of entries) or users who created or modified them.

An entry can link to more than one entry or asset, here's how:

• Multiple link fields, e.g. for the restaurant example, fields for fields.menu and fields.openingHours. You could even limit the entries a link field can point to by specifying link content type validation on the field.
• An array of links field, e.g. fields.menuItems in the restaurant's menu, representing an ordered list of related items.

Modeling attachments

Below is the example JSON structure of an entry with an asset attached:

{
  "fields": {
    "title": {
      "en-US": "Hello, World!"
    },
    "reference_field": {
      "en-US": [
        {"sys": {
          "type": "Link",
          "linkType": "Asset",
          "id": "id1"
        }},
        {"sys": {
          "type": "Link",
          "linkType": "Asset",
          "id": "id2"
        }}
        ...
      ]
    }
  }
}

For example, here's a restaurant with related photos:

{
  "sys": {
    "type": "Entry",
    "id": "il-doges"
  },
  "fields": {
    "mainPhoto": {"en-US":
      {"sys": {"type": "Link", "linkType": "Asset", "id": "such-doge"}
    },
    "photos": {"en-US": [
      {"sys": {"type": "Link", "linkType": "Asset", "id": "nice-food"}},
      {"sys": {"type": "Link", "linkType": "Asset", "id": "such-doge"}},
      {"sys": {"type": "Link", "linkType": "Asset", "id": "wow"}}
    ]}
  }
}

As with entry links, you can have multiple fields linking to a single fields.mainPhoto or multiple fields.photos assets.

You can limit the type of asset a link field can link to by specifying an asset file type validation on the field.

Link fields in content types

Adding links to an entry requires the entry's content type to have one or more link fields.

Here are some other examples, used in the context of a content type like this:

{
  "sys": {"type": "ContentType", "id": "restaurant"},
  "fields": [
    ...
  ]
}

Link field for entry

{
  "id": "menu",
  "type": "Link",
  "linkType": "Entry"
}

Link field for multiple entries

{
  "id": "menuItems",
  "type": "Array",
  "items": {
    "type": "Link",
    "linkType": "Entry"
  }
}

Link field for asset

{
  "id": "mainPhoto",
  "type": "Link",
  "linkType": "Asset"
}

Link field for multiple assets

{
  "id": "photos",
  "type": "Array",
  "items": {
    "type": "Link",
    "linkType": "Asset"
  }
}

Link values in entries

You can use link values in entries to specify actual links to other entries or assets. Before you can add these links you need to have added link fields to a content type.

Contentful represents link values as objects containing a sys property with the type and ID of the resource they're linking to:

Here are some examples, used in the context of an entry like this:

{
  "sys": {"type": "Entry", "id": "restaurant"},
  "fields": {
    "someField": {
      "en-US": ...
    }
  }
}

Link to an entry

{
  "sys": {
    "type": "Link",
    "linkType": "Entry",
    "id": "il-doges-nice-menu"
  }
}

Links to multiple entries

[
  {"sys": {"type": "Link", "linkType": "Entry", "id": "nice-burger"}},
  {"sys": {"type": "Link", "linkType": "Entry", "id": "such-dessert"}},
  {"sys": {"type": "Link", "linkType": "Entry", "id": "WOW"}}
]

Link to an asset

{
  "sys": {
    "type": "Link",
    "linkType": "Asset",
    "id": "such-doge"
  }
}

Links to multiple assets

[
  {"sys": {"type": "Link", "linkType": "Asset", "id": "nice-food"}},
  {"sys": {"type": "Link", "linkType": "Asset", "id": "such-doge"}},
  {"sys": {"type": "Link", "linkType": "Asset", "id": "wow"}}
]