Support NullsFirst for all databases.

Most databases order tables with the `NULL` value first, having it before
all other data values. Postgres has `NULLS` last.

Fortunately, ANSI SQL has an option to allow the database to specify where NULLS
come out in this sort order

    ORDER BY column ASC NULLS FIRST

MS SQL, SQLite, Oracle, and Postgres all follow this syntax. Unfortunately, MySql
does not.

Before:

PostgreSQL: both `.nulls_first()` and `.nulls_last()` work as designed.
Others: both raise a runtime error.

After:

MySQL: `.nulls_first()` works as designed.
MySQL: `.nulls_last()` raises a runtime error
Others: both work as designed

activerecord/CHANGELOG.md

@@ -1,3 +1,7 @@
+*   Add `.asc.nulls_first` for all databases. Unfortunately MySQL still doesn't like `nulls_last`.
+
+    *Keenan Brock*
+
 *   Improve performance of `one?` and `many?` by limiting the generated count query to 2 results.
 
     *Gonzalo Riestra*

activerecord/lib/arel/visitors/mysql.rb

@@ -58,6 +58,11 @@ def visit_Arel_Nodes_NotRegexp(o, collector)
           infix_value o, collector, " NOT REGEXP "
         end
 
+        # no-op
+        def visit_Arel_Nodes_NullsFirst(o, collector)
+          visit o.expr, collector
+        end
+
         # In the simple case, MySQL allows us to place JOINs directly into the UPDATE
         # query. However, this does not allow for LIMIT, OFFSET and ORDER. To support
         # these, we must use a subquery.

activerecord/lib/arel/visitors/postgresql.rb

@@ -78,16 +78,6 @@ def visit_Arel_Nodes_IsDistinctFrom(o, collector)
           visit o.right, collector
         end
 
-        def visit_Arel_Nodes_NullsFirst(o, collector)
-          visit o.expr, collector
-          collector << " NULLS FIRST"
-        end
-
-        def visit_Arel_Nodes_NullsLast(o, collector)
-          visit o.expr, collector
-          collector << " NULLS LAST"
-        end
-
         BIND_BLOCK = proc { |i| "$#{i}" }
         private_constant :BIND_BLOCK
 

activerecord/lib/arel/visitors/to_sql.rb

@@ -359,6 +359,17 @@ def visit_Arel_Nodes_Descending(o, collector)
           visit(o.expr, collector) << " DESC"
         end
 
+        # NullsFirst is available on all but MySQL, where it is redefined.
+        def visit_Arel_Nodes_NullsFirst(o, collector)
+          visit o.expr, collector
+          collector << " NULLS FIRST"
+        end
+
+        def visit_Arel_Nodes_NullsLast(o, collector)
+          visit o.expr, collector
+          collector << " NULLS LAST"
+        end
+
         def visit_Arel_Nodes_Group(o, collector)
           visit o.expr, collector
         end

activerecord/test/cases/arel/visitors/mysql_test.rb

@@ -152,6 +152,15 @@ def compile(node)
           }
         end
       end
+
+      describe "Nodes::Ordering" do
+        it "should no-op ascending nulls first" do
+          test = Table.new(:users)[:first_name].asc.nulls_first
+          _(compile(test)).must_be_like %{
+            "users"."first_name" ASC
+          }
+        end
+      end
     end
   end
 end

activerecord/test/cases/arel/visitors/postgres_test.rb

@@ -316,36 +316,6 @@ def compile(node)
         end
       end
 
-      describe "Nodes::Ordering" do
-        it "should handle nulls first" do
-          test = Table.new(:users)[:first_name].desc.nulls_first
-          _(compile(test)).must_be_like %{
-            "users"."first_name" DESC NULLS FIRST
-          }
-        end
-
-        it "should handle nulls last" do
-          test = Table.new(:users)[:first_name].desc.nulls_last
-          _(compile(test)).must_be_like %{
-            "users"."first_name" DESC NULLS LAST
-          }
-        end
-
-        it "should handle nulls first reversed" do
-          test = Table.new(:users)[:first_name].desc.nulls_first.reverse
-          _(compile(test)).must_be_like %{
-            "users"."first_name" ASC NULLS LAST
-          }
-        end
-
-        it "should handle nulls last reversed" do
-          test = Table.new(:users)[:first_name].desc.nulls_last.reverse
-          _(compile(test)).must_be_like %{
-            "users"."first_name" ASC NULLS FIRST
-          }
-        end
-      end
-
       describe "Nodes::InfixOperation" do
         it "should handle Contains" do
           inner = Nodes.build_quoted('{"foo":"bar"}')

activerecord/test/cases/arel/visitors/to_sql_test.rb

@@ -382,6 +382,34 @@ def dispatch
             "users"."id" DESC
           }
         end
+
+        it "should handle nulls first" do
+          node = @attr.desc.nulls_first
+          _(compile(node)).must_be_like %{
+            "users"."id" DESC NULLS FIRST
+          }
+        end
+
+        it "should handle nulls last" do
+          node = @attr.desc.nulls_last
+          _(compile(node)).must_be_like %{
+            "users"."id" DESC NULLS LAST
+          }
+        end
+
+        it "should handle nulls first reversed" do
+          node = @attr.desc.nulls_first.reverse
+          _(compile(node)).must_be_like %{
+            "users"."id" ASC NULLS LAST
+          }
+        end
+
+        it "should handle nulls last reversed" do
+          node = @attr.desc.nulls_last.reverse
+          _(compile(node)).must_be_like %{
+            "users"."id" ASC NULLS FIRST
+          }
+        end
       end
 
       describe "Nodes::In" do