Refactor QueryBuilder and DbRequestBuilder

- Refactor ORDER BY and LIMIT query fragments
- Move ReadRequest building to DbRequestBuilder
- Clarify Relation link table attributes
- Change Join Operation to JoinCond

src/PostgREST/Types.hs

@@ -149,9 +149,10 @@ data Relation = Relation {
 , relFTable   :: Table
 , relFColumns :: [Column]
 , relType     :: RelationType
-, relLTable   :: Maybe Table
-, relLCols1   :: Maybe [Column]
-, relLCols2   :: Maybe [Column]
+-- The Link attrs are used when RelationType == Many
+, relLinkTable   :: Maybe Table
+, relLinkCols1   :: Maybe [Column]
+, relLinkCols2   :: Maybe [Column]
 } deriving (Show, Eq)
 
 -- | Cached attributes of a JSON payload
@@ -215,8 +216,7 @@ ftsOperators = M.fromList [
 data OpExpr = OpExpr Bool Operation deriving (Eq, Show)
 data Operation = Op Operator SingleVal |
                  In ListVal |
-                 Fts Operator (Maybe Language) SingleVal |
-                 Join QualifiedIdentifier ForeignKey deriving (Eq, Show)
+                 Fts Operator (Maybe Language) SingleVal deriving (Eq, Show)
 type Language = Text
 
 -- | Represents a single value in a filter, e.g. id=eq.singleval
@@ -274,8 +274,9 @@ type SelectItem = (Field, Maybe Cast, Maybe Alias, Maybe RelationDetail)
 -- | Path of the embedded levels, e.g "clients.projects.name=eq.." gives Path ["clients", "projects"]
 type EmbedPath = [Text]
 data Filter = Filter { field::Field, opExpr::OpExpr } deriving (Show, Eq)
+data JoinCond = JoinCond (QualifiedIdentifier, FieldName) (QualifiedIdentifier, FieldName) deriving (Show, Eq)
 
-data ReadQuery = Select { select::[SelectItem], from::[TableName], where_::[LogicTree], order::Maybe [OrderTerm], range_::NonnegRange } deriving (Show, Eq)
+data ReadQuery = Select { select::[SelectItem], from::[TableName], where_::[LogicTree], joinConds::[JoinCond], order::[OrderTerm], range_::NonnegRange } deriving (Show, Eq)
 data MutateQuery = Insert { in_::TableName, insPkCols::[Text], qPayload::PayloadJSON, onConflict:: Maybe PreferResolution, where_::[LogicTree], returning::[FieldName] }
                  | Delete { in_::TableName, where_::[LogicTree], returning::[FieldName] }
                  | Update { in_::TableName, qPayload::PayloadJSON, where_::[LogicTree], returning::[FieldName] } deriving (Show, Eq)