Adding cross-references

Overview

Teaching: 3 min
Exercises: 4 min

Questions

• How can I describe an entity further?

• How can I cross-reference different entities?

Objectives

• Understand cross-references in flattened JSON-LD

• Add a data entity reference from the root entity

• Add another type to the root entity

About cross-references

In a RO-Crate Metadata Document, entities are cross-referenced using @id reference objects, rather than using deeply nested JSON objects. In short, this flattened JSON-LD style (shown below) allows any entity to reference any other entity, and RO-Crate consumers can directly find all the descriptions of a given entity as a single JSON object.

Add cross-reference to data entity

Consider the root Data Entity ./, and add such a cross-reference to the file data.csv using the property called hasPart:

Solution

{
    "@id": "./",
    "@type": "Dataset",
    "hasPart": [ 
      {"@id": "data.csv"} 
    ],
    "…": "…"
}

The RO-Crate root is always typed Dataset, though @type may in some cases have additional types by using a JSON array instead of a single value. Most entities can have such more specific types, e.g. chosen from schema.org type list.

Add an additional type

1. Navigate the schema.org type list to find a subtype of CreativeWork that is suitable for a learning resource.
2. Modify the root entity’s @type to be an array.
3. Add the type name for learning resource at the end of the array.

Solution

{
    "@id": "./",
    "@type": ["Dataset", "LearningResource"],
    "hasPart": [ 
      {"@id": "data.csv"} 
    ],
    "…": "…"
}

The root has several metadata properties that describe the RO-Crate as a whole, considering it as a Research Object of collected resources. The section on root data entity details further the required and recommended properties of the root ./.

Key Points

• The @id uniquely identifies the entity within the RO-Crate

• The @id key is used for cross-referencing

• Multiple types can be listed by using an array