Adding cross-references

Estimated time: 7 minutes

Overview

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

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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.

JSON block with id ro-crate-metadata.json has some attributes, conformsTo RO-Crate 1.2, and about referencing id ./. In second JSON block with id ./ we see additional attributes such as its name and description.

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:

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JSON

{
   "@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.

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JSON

{
   "@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