refactor: rename RelationType to Cardinality

Remove Root type constructor

src/PostgREST/DbRequestBuilder.hs

@@ -110,9 +110,7 @@ addRelations schema allRelations parentNode (Node (query@Select{from=tbl}, (node
                 findRelation schema allRelations nodeName parentNodeTable relationDetail in
       Node <$> newReadNode <*> (updateForest . hush $ Node <$> newReadNode <*> pure forest)
     _ ->
-      let rn = (query, (nodeName, Just r, alias, Nothing, depth))
+      let rn = (query, (nodeName, Nothing, alias, Nothing, depth)) in
-          r = Relation t [] t [] Root Nothing Nothing Nothing
-          t = Table schema nodeName Nothing True in -- !!! TODO find another way to get the table from the query
       Node rn <$> updateForest (Just $ Node rn forest)
   where
     updateForest :: Maybe ReadRequest -> Either ApiRequestError [ReadRequest]
@@ -201,13 +199,16 @@ findRelation schema allRelations nodeTableName parentNodeTableName relationDetai
 addJoinConditions :: Maybe Alias -> ReadRequest -> Either ApiRequestError ReadRequest
 addJoinConditions previousAlias (Node node@(query@Select{from=tbl}, nodeProps@(_, relation, _, _, depth)) forest) =
   case relation of
-    Just Relation{relType=Root} -> Node node <$> updatedForest -- this is the root node
     Just rel@Relation{relType=Parent} -> Node (augmentQuery rel, nodeProps) <$> updatedForest
     Just rel@Relation{relType=Child} -> Node (augmentQuery rel, nodeProps) <$> updatedForest
-    Just rel@Relation{relType=Many, relLinkTable=(Just linkTable)} ->
+    Just rel@Relation{relType=Many, relLinkTable=lTable} ->
+      case lTable of
+        Just linkTable ->
           let rq = augmentQuery rel in
           Node (rq{implicitJoins=tableQi linkTable:implicitJoins rq}, nodeProps) <$> updatedForest
-    _ -> Left UnknownRelation
+        Nothing ->
+          Left UnknownRelation
+    Nothing -> Node node <$> updatedForest
   where
     newAlias = case isSelfJoin <$> relation of
       Just True
@@ -232,7 +233,6 @@ getJoinConditions previousAlias newAlias (Relation Table{tableSchema=tSchema, ta
     Many   ->
         let ltN = maybe "" tableName lt in
         zipWith (toJoinCondition tN ltN) cols (fromMaybe [] lc1) ++ zipWith (toJoinCondition ftN ltN) fCols (fromMaybe [] lc2)
-    Root   -> witness
   where
     toJoinCondition :: Text -> Text -> Column -> Column -> JoinCondition
     toJoinCondition tb ftb c fc =
@@ -355,7 +355,6 @@ returningCols rr@(Node _ forest) = returnings
           Child  -> Just cols
           Many   -> Just cols
         _ -> Nothing
-        _ -> Nothing
       ) forest
     -- However if the "client_id" is present, e.g. mutateRequest to /projects?select=client_id,name,clients(name)
     -- we would get `RETURNING client_id, name, client_id` and then we would produce the "column reference \"client_id\" is ambiguous"

src/PostgREST/QueryBuilder.hs

@@ -73,9 +73,7 @@ getJoinsSelects rr@(Node (_, (name, Just Relation{relType=relTyp,relTable=Table{
              <> "FROM (" <> subquery <> ") " <> pgFmtIdent table
              <> "), '[]') AS " <> pgFmtIdent (fromMaybe name alias) in
       (j,sel:s)
---readRequestToQuery is called only after addJoinConditions which ensures the only posible relations are Child Parent Many
+getJoinsSelects (Node (_, (_, Nothing, _, _, _)) _) _ = ([], [])
-    Root -> witness
-getJoinsSelects _ _ = witness
 
 mutateRequestToQuery :: MutateRequest -> SqlQuery
 mutateRequestToQuery (Insert mainQi iCols onConflct putConditions returnings) =

src/PostgREST/Types.hs

@@ -257,7 +257,12 @@ data QualifiedIdentifier = QualifiedIdentifier {
 } deriving (Show, Eq, Ord)
 
 
-data RelationType = Child | Parent | Many | Root deriving (Show, Eq)
+-- | The relationship [cardinality](https://en.wikipedia.org/wiki/Cardinality_(data_modeling)).
+-- | TODO: missing one-to-one
+data Cardinality = Child  -- ^ a.k.a. many-to-one
+                 | Parent -- ^ a.k.a. one-to-many
+                 | Many   -- ^ a.k.a. many-to-many
+                 deriving (Show, Eq)
 
 {-|
   The name 'Relation' here is used with the meaning
@@ -272,15 +277,15 @@ data Relation = Relation {
 , relColumns   :: [Column]
 , relFTable    :: Table
 , relFColumns  :: [Column]
-, relType      :: RelationType
+, relType      :: Cardinality
--- The Link attrs are used when RelationType == Many
+-- The Link attrs are used when Cardinality == Many
 , relLinkTable :: Maybe Table
 , relLinkCols1 :: Maybe [Column]
 , relLinkCols2 :: Maybe [Column]
 } deriving (Show, Eq)
 
 isSelfJoin :: Relation -> Bool
-isSelfJoin r = relType r /= Root && relTable r == relFTable r
+isSelfJoin r = relTable r == relFTable r
 
 data PayloadJSON =
   -- | Cached attributes of a JSON payload