Pass AR tests by moving DISTINCT to GROUP BY in windowed SQL.

* Tests now passing.
  test_count_eager_with_has_many_and_limit_and_high_offset
  test_eager_with_has_many_and_limit_and_high_offset

* SQL like this:

  SELECT DISTINCT TOP (2) [__rnt].id
  FROM (
    SELECT ROW_NUMBER() OVER (ORDER BY [posts].[id] ASC) AS [__rn], [posts].id
    FROM [posts]
    LEFT OUTER JOIN [authors] ON [authors].[id] = [posts].[author_id]
    LEFT OUTER JOIN [comments] ON [comments].[post_id] = [posts].[id]
    WHERE (authors.name = N'David')
  ) AS [__rnt]
  WHERE [__rnt].[__rn] > (10)

  Now like this:

  SELECT TOP (2) [__rnt].id
  FROM (
    SELECT ROW_NUMBER() OVER (ORDER BY [posts].[id] ASC) AS [__rn], [posts].id
    FROM [posts]
    LEFT OUTER JOIN [authors] ON [authors].[id] = [posts].[author_id]
    LEFT OUTER JOIN [comments] ON [comments].[post_id] = [posts].[id]
    WHERE (authors.name = N'David')
    GROUP BY [posts].id
  ) AS [__rnt]
  WHERE [__rnt].[__rn] > (10)

Author: metaskills

CHANGELOG

@@ -1,4 +1,11 @@
 
+* master *
+
+* Pass AR tests by moving DISTINCT to GROUP BY in windowed SQL.
+  - test_count_eager_with_has_many_and_limit_and_high_offset
+  - test_eager_with_has_many_and_limit_and_high_offset
+
+
 * 3.1.3 *
 
 * Distinguish between identity and primary key key columns during schema reflection. Allows us 

lib/arel/visitors/sqlserver.rb

@@ -138,6 +138,7 @@ def visit_Arel_Nodes_SelectStatementWithOutOffset(o, windowed=false)
         orders = o.orders.uniq
         if windowed
           projections = function_select_statement?(o) ? projections : projections.map { |x| projection_without_expression(x) }
+          groups = projections.map { |x| projection_without_expression(x) } if windowed_single_distinct_select_statement?(o) && groups.empty?
         elsif eager_limiting_select_statement?(o)
           groups = projections.map { |x| projection_without_expression(x) }
           projections = projections.map { |x| projection_without_expression(x) }
@@ -162,7 +163,7 @@ def visit_Arel_Nodes_SelectStatementWithOutOffset(o, windowed=false)
       def visit_Arel_Nodes_SelectStatementWithOffset(o)
         orders = rowtable_orders(o)
         [ "SELECT",
-          (visit(o.limit) if o.limit && !single_distinct_select_statement?(o)),
+          (visit(o.limit) if o.limit && !windowed_single_distinct_select_statement?(o)),
           (rowtable_projections(o).map{ |x| visit(x) }.join(', ')),
           "FROM (",
             "SELECT ROW_NUMBER() OVER (ORDER BY #{orders.map{ |x| visit(x) }.join(', ')}) AS [__rn],",
@@ -238,6 +239,10 @@ def single_distinct_select_statement?(o)
             p1.respond_to?(:include?) && p1.include?('DISTINCT'))
       end
 
+      def windowed_single_distinct_select_statement?(o)
+        o.limit && o.offset && single_distinct_select_statement?(o)
+      end
+      
       def single_distinct_select_everything_statement?(o)
         single_distinct_select_statement?(o) && visit(o.cores.first.projections.first).ends_with?(".*")
       end
@@ -316,7 +321,17 @@ def find_and_fix_uncorrelated_joins_in_select_statement(o)
 
       def rowtable_projections(o)
         core = o.cores.first
-        if single_distinct_select_statement?(o)
+        if windowed_single_distinct_select_statement?(o) && core.groups.blank?
+          tn = table_from_select_statement(o).name
+          core.projections.map do |x|
+            x.dup.tap do |p|
+              p.sub! 'DISTINCT', ''
+              p.insert 0, visit(o.limit) if o.limit
+              p.gsub! /\[?#{tn}\]?\./, '[__rnt].'
+              p.strip!
+            end
+          end
+        elsif single_distinct_select_statement?(o)
           tn = table_from_select_statement(o).name
           core.projections.map do |x|
             x.dup.tap do |p|

test/cases/pessimistic_locking_test_sqlserver.rb

@@ -65,7 +65,7 @@ def setup
     end
 
     should 'cope with eager loading un-locked paginated' do
-      eager_ids_sql = /SELECT DISTINCT TOP \(5\).*FROM \[people\] WITH \(NOLOCK\)/
+      eager_ids_sql = /SELECT TOP \(5\).*FROM \[people\] WITH \(NOLOCK\)/
       loader_sql = /FROM \[people\] WITH \(NOLOCK\).*WHERE \[people\]\.\[id\] IN/
       assert_sql(eager_ids_sql,loader_sql) do
         Person.all(:include => :readers, :lock => 'WITH (NOLOCK)', :limit => 5, :offset => 10)