Mapping between schema.org and Wikidata Properties

[malajvan]

I want to report my experience related to the mapping between schema.org properties and Wikidata properties. I came across these while overriding the context for musicbrainz which is currently using the schema.org context for jsonld here. Even though most common properties are linked, there are some missing links and quirks that I think would be beneficial to report here.

Official(?) mapping effort:

There exists a Wikidata's WikiProject to create a mapping between these two schemas, and it's available here: Wikidata:WikiProject/Wikidata-Schema.org mapping.

Some problems:

1. Missing Connections:

Some existing correlations are absent from the official mapping document, despite existing in the Wikidata property database. For instance, the mapping for schema.org/image (linked to Wikidata property P18) is missing.

However, there are some obvious equivalences between the 2 sources that are not linked at all (record label (P264) and https://schema.org/recordLabel)

2. Non-Direct Mappings:

Certain mappings between schema.org and Wikidata are not straightforward. For example, schema.org/releaseDate does not have a direct mapping with Wikidata. Instead, it is linked to schema.org/publicationDate and Wikidata property P577, which represents publication date.
This makes the reconciled predicates slightly different from the releaseDate as chosen in MusicBrainz if we choose to reconcile releaseDate with P577.

3. Unusual Mappings:

Some mappings seems weird (to me, at least). For instance, a contentURL from schema.org mapping is associated with "URL (P2699) with qualifier of (P642) pointing to download (Q7126717)"
I'm unsure how this works, and currently just putting P2699 for the musicbrainz jsonld

4. Qs vs. Ps:

It has been observed that certain schema.org properties are linked to Q-values (entities) rather than P-values (properties). For instance, thumbnail (Q873806),has corresponding thumbnail properties (Ps) in schema.org.
I need to look into this more to understand if this is deliberate