Add undocumented as=_explain to inspect the filtered query

api/src/handlers/search-runner.js

@@ -65,10 +65,24 @@ const doSearch = async (event, searchParams = {}) => {
     .authFilter(event)
     .toJson();
 
+  const searchModels = modelsToTargets(models);
+  const searchQuery = sanitizeQueryString(event.queryStringParameters);
+
+  if (format === "_explain") {
+    return {
+      statusCode: 200,
+      body: JSON.stringify({
+        models: searchModels,
+        body: JSON.parse(filteredSearchContext),
+        query: searchQuery,
+      }),
+    };
+  }
+
   const esResponse = await search(
-    modelsToTargets(models),
+    searchModels,
     filteredSearchContext,
-    sanitizeQueryString(event.queryStringParameters)
+    searchQuery
   );
 
   return await responseTransformer.transformSearchResult(esResponse, pager);

api/test/integration/search.test.js

@@ -262,5 +262,25 @@ describe("Search routes", () => {
         "?_source_includes=title%2Caccession_number"
       );
     });
+
+    it("returns the query when as=_explain is specified", async () => {
+      const originalQuery = {
+        query: { query_string: { query: "*" } },
+      };
+      const event = helpers
+        .mockEvent("GET", "/search")
+        .queryParams({ as: "_explain" })
+        .render();
+      const authQuery = new RequestPipeline(originalQuery)
+        .authFilter(helpers.preprocess(event))
+        .toJson();
+      const expectedQuery = JSON.parse(authQuery);
+
+      const result = await handler(event);
+      expect(result.statusCode).to.eq(200);
+      const resultBody = JSON.parse(result.body);
+      expect(resultBody.models).to.eq("dc-v2-work");
+      expect(resultBody.body).to.deep.equal(expectedQuery);
+    });
   });
 });