If you are running a copy of Wikibase, we have split out data model code from WikibaseLib into a separate extension. (which can be used independent of MediaWiki)
You need to have the WikibaseDataModel extension in the extensions directory. It will automatically be loaded by Wikibase.
If you are missing WikibaseDataModel, then you may see an error:
Fatal error: Class 'Wikibase\EntityId' not found
Additionally, the WikibaseQuery, WikibaseQueryEngine and WikibaseDatabase components have been split into separate extensions. These are only needed for the newest experimental query stuff.
To make things easier, Wikibase has a composer.json file to manage the dependencies and you can use that, if you like.
-- Katie Filbert
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