From d067c03130595dad67bffb61898898049ddfb344 Mon Sep 17 00:00:00 2001
From: collerek <collerek@gmail.com>
Date: Sun, 7 Mar 2021 17:37:32 +0100
Subject: [PATCH] try with min and max filters depending on direction

---
 ormar/queryset/actions/order_action.py |  7 ++++++
 ormar/queryset/query.py                | 31 +++++++++++++-------------
 2 files changed, 22 insertions(+), 16 deletions(-)

diff --git a/ormar/queryset/actions/order_action.py b/ormar/queryset/actions/order_action.py
index 28b92d1..f999f32 100644
--- a/ormar/queryset/actions/order_action.py
+++ b/ormar/queryset/actions/order_action.py
@@ -46,6 +46,13 @@ class OrderAction(QueryAction):
         prefix = f"{self.table_prefix}_" if self.table_prefix else ""
         return f"{prefix}{self.table}" f".{self.field_alias}"
 
+    def get_min_or_max(self):
+        prefix = f"{self.table_prefix}_" if self.table_prefix else ""
+        if self.direction == '':
+            return text(f"min({prefix}{self.table}" f".{self.field_alias})")
+        else:
+            return text(f"max({prefix}{self.table}" f".{self.field_alias}) desc")
+
     def get_text_clause(self) -> sqlalchemy.sql.expression.TextClause:
         """
         Escapes characters if it's required.
diff --git a/ormar/queryset/query.py b/ormar/queryset/query.py
index 8e7db64..3a3e96c 100644
--- a/ormar/queryset/query.py
+++ b/ormar/queryset/query.py
@@ -171,32 +171,31 @@ class Query:
         """
         pk_alias = self.model_cls.get_column_alias(self.model_cls.Meta.pkname)
         pk_aliased_name = f"{self.table.name}.{pk_alias}"
-        qry_text = sqlalchemy.text(f"{pk_aliased_name} as limit_column")
-        maxes = dict()
-        for ind, order in enumerate(list(self.sorted_orders.keys())):
+        qry_text = sqlalchemy.text(f"{pk_aliased_name}")
+        maxes = OrderedDict()
+        for order in list(self.sorted_orders.keys()):
             if order is not None and order.get_field_name_text() != pk_aliased_name:
                 aliased_col = order.get_field_name_text()
-                maxes[aliased_col] = sqlalchemy.text(f"{aliased_col} as col{ind}")
+                maxes[aliased_col] = order.get_min_or_max()
+            elif order.get_field_name_text() == pk_aliased_name:
+                maxes[pk_aliased_name] = order.get_text_clause()
 
-        limit_qry = sqlalchemy.sql.select([qry_text] + list(maxes.values()))
+        limit_qry = sqlalchemy.sql.select([qry_text])
         limit_qry = limit_qry.select_from(self.select_from)
-        limit_qry = self._apply_expression_modifiers(limit_qry)
         limit_qry = FilterQuery(filter_clauses=self.filter_clauses).apply(limit_qry)
         limit_qry = FilterQuery(filter_clauses=self.exclude_clauses,
                                 exclude=True).apply(
             limit_qry
         )
-        limit_qry = OrderQuery(sorted_orders=self.sorted_orders).apply(limit_qry)
-        limit_qry = limit_qry.alias("inner_limit_query")
-        outer_text = sqlalchemy.text(f"distinct limit_column")
-        outer_qry = sqlalchemy.sql.select([outer_text])
-        outer_qry = outer_qry.select_from(limit_qry)
-        outer_qry = LimitQuery(limit_count=self.limit_count).apply(outer_qry)
-        outer_qry = OffsetQuery(query_offset=self.query_offset).apply(outer_qry)
-        outer_qry = outer_qry.alias("limit_query")
+        limit_qry = limit_qry.group_by(qry_text)
+        for order_by in maxes.values():
+            limit_qry = limit_qry.order_by(order_by)
+        limit_qry = LimitQuery(limit_count=self.limit_count).apply(limit_qry)
+        limit_qry = OffsetQuery(query_offset=self.query_offset).apply(limit_qry)
+        limit_qry = limit_qry.alias("limit_query")
         on_clause = sqlalchemy.text(
-            f"limit_query.limit_column={self.table.name}.{pk_alias}")
-        return outer_qry, on_clause
+            f"limit_query.{pk_alias}={self.table.name}.{pk_alias}")
+        return limit_qry, on_clause
 
     def _apply_expression_modifiers(
             self, expr: sqlalchemy.sql.select