Added mandatory query tests, with some examples for the structure end…

…point

optimade/validator/validator.py

@@ -34,6 +34,15 @@
 LINKS_ENDPOINT = "links"
 REQUIRED_ENTRY_ENDPOINTS = ["references", "structures"]
 
+ENDPOINT_MANDATORY_QUERIES = {
+    "structures": [
+        'elements HAS "Na"',
+        'elements HAS ANY "Na", "Cl"',
+        'elements HAS ALL "Na", "Cl"',
+    ],
+    "references": [],
+}
+
 RESPONSE_CLASSES = {
     "references": ValidatorReferenceResponseMany,
     "references/": ValidatorReferenceResponseOne,
@@ -317,6 +326,12 @@ def main(self):
             self._log.debug("Testing single entry request of type %s", endp)
             self.test_single_entry_endpoint(endp)
 
+        for endp in ENDPOINT_MANDATORY_QUERIES:
+            # skip empty endpoint query lists
+            if ENDPOINT_MANDATORY_QUERIES[endp]:
+                self._log.debug("Testing mandatory query syntax on endpoint %s", endp)
+                self.test_mandatory_query_syntax(endp, ENDPOINT_MANDATORY_QUERIES[endp])
+
         self._log.debug("Testing %s endpoint", LINKS_ENDPOINT)
         self.test_info_or_links_endpoints(LINKS_ENDPOINT)
 
@@ -491,3 +506,21 @@ def get_endpoint(self, request_str):
                 f"Request to '{request_str}' returned HTTP code: {response.status_code}"
             )
         return response, "request successful."
+
+    def test_mandatory_query_syntax(self, endpoint, endpoint_queries):
+        """ Perform a list of valid queries and assert that no errors are raised.
+
+        Parameters:
+            endpoint (str): the endpoint to query (e.g. "structures").
+            endpoint_queries (list): the list of valid mandatory queries
+                for that endpoint, where the queries do not include the
+                "?filter=" prefix, e.g. ['elements HAS "Na"'].
+
+        """
+
+        valid_queries = [f"{endpoint}?filter={query}" for query in endpoint_queries]
+        query_test_cases = [
+            test_case(self.get_endpoint(query)) for query in valid_queries
+        ]
+        for case in query_test_cases:
+            case()