fix: empty spread embeddings return unexpected SQL error

Fixes the SQL error from postgres when an empty spread embeddings
like `...table()` is requested.

CHANGELOG.md

@@ -16,6 +16,7 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 
 - Fix OpenAPI broken docs link by @taimoorzaeem in #4080
 - Fix OpenAPI specification incorrectly exposing GET methods for volatile functions by @joelonsql in #4174
+- Fix empty spread embeddings return unexpected SQL error by @taimoorzaeem in #3887
 
 ### Changed
 

src/PostgREST/Query/QueryBuilder.hs

@@ -69,21 +69,21 @@ readPlanToQuery node@(Node ReadPlan{select,from=mainQi,fromAlias,where_=logicFor
 
 getJoinSelects :: ReadPlanTree -> [SQL.Snippet]
 getJoinSelects (Node ReadPlan{relSelect} _) =
-  mapMaybe relSelectToSnippet relSelect
+  join $ map relSelectToSnippet relSelect
   where
-    relSelectToSnippet :: RelSelectField -> Maybe SQL.Snippet
+    relSelectToSnippet :: RelSelectField -> [SQL.Snippet]
     relSelectToSnippet fld =
       let aggAlias = pgFmtIdent $ rsAggAlias fld
       in
         case fld of
           JsonEmbed{rsEmptyEmbed = True} ->
-            Nothing
+            []
           JsonEmbed{rsSelName, rsEmbedMode = JsonObject} ->
-            Just $ "row_to_json(" <> aggAlias <> ".*)::jsonb AS " <> pgFmtIdent rsSelName
+            ["row_to_json(" <> aggAlias <> ".*)::jsonb AS " <> pgFmtIdent rsSelName]
           JsonEmbed{rsSelName, rsEmbedMode = JsonArray} ->
-            Just $ "COALESCE( " <> aggAlias <> "." <> aggAlias <> ", '[]') AS " <> pgFmtIdent rsSelName
+            ["COALESCE( " <> aggAlias <> "." <> aggAlias <> ", '[]') AS " <> pgFmtIdent rsSelName]
           Spread{rsSpreadSel, rsAggAlias} ->
-            Just $ intercalateSnippet ", " (pgFmtSpreadSelectItem rsAggAlias <$> rsSpreadSel)
+            pgFmtSpreadSelectItem rsAggAlias <$> rsSpreadSel
 
 getJoins :: ReadPlanTree -> [SQL.Snippet]
 getJoins (Node _ []) = []

test/spec/Feature/Query/SpreadQueriesSpec.hs

@@ -600,3 +600,37 @@ spec =
           { matchStatus  = 200
           , matchHeaders = [matchContentTypeJson]
           }
+
+    context "empty spreads embeds" $
+      it "should work return the same as empty embeddings" $ do
+        get "/actors?select=*,...films()"
+          `shouldRespondWith`
+          [json| [{"id":1,"name":"john"}, {"id":2,"name":"mary"}] |]
+          { matchStatus  = 200
+          , matchHeaders = [matchContentTypeJson]
+          }
+
+        get "/grandchild_entities?select=name,...child_entities(parent_name:name,...entities())"
+          `shouldRespondWith`
+          [json|
+            [{"name":"grandchild entity 1","parent_name":"child entity 1"},
+             {"name":"grandchild entity 2","parent_name":"child entity 1"},
+             {"name":"grandchild entity 3","parent_name":"child entity 2"},
+             {"name":"(grandchild,entity,4)","parent_name":"child entity 2"},
+             {"name":"(grandchild,entity,5)","parent_name":"child entity 2"}]
+          |]
+          { matchStatus  = 200
+          , matchHeaders = [matchContentTypeJson]
+          }
+
+        get "/factories?select=factory:name,...processes()"
+          `shouldRespondWith`
+          [json|
+            [{"factory":"Factory A"},
+             {"factory":"Factory B"},
+             {"factory":"Factory C"},
+             {"factory":"Factory D"}]
+          |]
+          { matchStatus  = 200
+          , matchHeaders = [matchContentTypeJson]
+          }