Fix anykeyh#21

spec/model/model_spec.cr

@@ -247,6 +247,17 @@ module ModelSpec
         end
       end
 
+      it "can count using group_by" do
+        temporary do
+          reinit
+          9.times do |x|
+            User.create!({first_name: "user#{x}", last_name: "Doe"})
+          end
+
+          User.query.group_by("last_name").count.should eq(1)
+        end
+      end
+
       it "can find_or_create" do
         temporary do
           reinit

src/clear/sql/query/aggregate.cr

@@ -3,18 +3,20 @@ module Clear::SQL::Query::Aggregate
   def count(type : X.class = Int64) forall X
     # save the `select` column clause to ensure non-mutability of the query
     columns = @columns.dup
-    o = if (@offset || @limit)
-          X.new(Clear::SQL.select("COUNT(*)").from({query_count: self.clear_select.select("1")}).scalar(Int64))
+    o = if (@offset || @limit || @group_bys)
+          subquery = self.dup.clear_order_bys.clear_select.select("1")
+          X.new(Clear::SQL.select("COUNT(*)").from({query_count: subquery}).scalar(Int64))
         else
-          X.new(self.clear_select.select("COUNT(*)").scalar(Int64))
+          new_query = self.dup.clear_select.select("COUNT(*)")
+          X.new(new_query.scalar(Int64))
         end
     @columns = columns
 
     return o
   end
 
   # Call an custom aggregation function, like MEDIAN or other
-  # Note than COUNT, MIN, MAX and AVG are conveniently mapped.
+  # Note than COUNT, MIN, MAX and AVG are already conveniently mapped.
   def agg(field, x : X.class) forall X
     self.clear_select.select(field).scalar(X)
   end