The query syntax for metadata server follows a client directed query methodology, whereby the consumer of the API provides a detailed specification of the query it wants to execute rather than adhering to a very strict REST template. This provides flexibility to the client but puts a lot of responsibility on the client developer to write efficient queries.
Query document
MDS uses the idea of a query document to specify queries, rather than a more traditional key-value filter or DQL (e.g. SQL) format. The query documents are JSON format and are provided in the filter parameter of an API request.
Equality
Simple equality is expressed through query-by-example semantics; for example, to get the object with id "1234-5678":
?filter={"id": "1234-5678"}
Sub-object reference
If the API being queried supports sub-objects, you can reference the fields of the sub-object using simple dot notation:
?filter={"editorial.id": "1234-5678"}
Conjunction (AND)
A comma-list of conditions is naturally joined as an AND:
?filter={"editorial.id": "1234", "technical.id": "5678"}
For much more complex queries, you can explicitly write this with an $and:
?filter={"$and": [{"editorial.id": "1234"}, {"technical.id": "5678"}]}
Disjunction (OR)
Similar to $and:
?filter={"$or": [{"editorial.id": "1234"}, {"technical.id": "5678"}]}
Negation (NOT)
To apply an inverse condition:
?filter={$ne": {"editorial.id": "1234-5678"}}
Ranges
Greater than ($gt), less than ($lt), greater than or equal to ($gte), and less than or equal to ($lte), can be applied as follows:
?filter={"period.start": {"$gte": 1000, "$lt": 2000}}
In
Allows you to match on a set of values:
?filter={"editorial.id": {"$in": ["1234", "5678"]}
Exists
Returns if a field exists (or not):
?filter={"editorial.id": {"$exists": true}}