Extract SQL logic from Arel::Relation into compilers.

3 files changed, 49 insertions, 0 deletions

diff --git a/lib/arel/engines/sql/compilers/mysql_compiler.rb b/lib/arel/engines/sql/compilers/mysql_compiler.rb
new file mode 100644
index 0000000000..e3cf1f2add
--- /dev/null
+++ b/lib/arel/engines/sql/compilers/mysql_compiler.rb
@@ -0,0 +1,7 @@
+module Arel
+  module SqlCompiler
+    class MySQLCompiler < GenericCompiler
+    end
+  end
+end
+
diff --git a/lib/arel/engines/sql/compilers/postgresql_compiler.rb b/lib/arel/engines/sql/compilers/postgresql_compiler.rb
new file mode 100644
index 0000000000..c3360b53a5
--- /dev/null
+++ b/lib/arel/engines/sql/compilers/postgresql_compiler.rb
@@ -0,0 +1,33 @@
+module Arel
+  module SqlCompiler
+    class PostgreSQLCompiler < GenericCompiler
+
+      def select_sql
+        if !orders.blank? && using_distinct_on?
+          # PostgreSQL does not allow arbitrary ordering when using DISTINCT ON, so we work around this
+          # by wrapping the +sql+ string as a sub-select and ordering in that query.
+          order = order_clauses.join(', ').split(',').map { |s| s.strip }.reject(&:blank?)
+          order = order.zip((0...order.size).to_a).map { |s,i| "id_list.alias_#{i} #{'DESC' if s =~ /\bdesc$/i}" }.join(', ')
+
+          query = build_query \
+            "SELECT #{select_clauses.kind_of?(::Array) ? select_clauses.join("") : select_clauses.to_s}",
+            "FROM #{from_clauses}",
+            (joins(self) unless joins(self).blank? ),
+            ("WHERE #{where_clauses.join(" AND ")}" unless wheres.blank? ),
+            ("GROUP BY #{group_clauses.join(', ')}" unless groupings.blank? ),
+            ("HAVING #{having_clauses.join(', ')}" unless havings.blank? ),
+            ("#{locked}" unless locked.blank? )
+
+          build_query \
+            "SELECT * FROM (#{query}) AS id_list",
+            "ORDER BY #{order}",
+            ("LIMIT #{taken}" unless taken.blank? ),
+            ("OFFSET #{skipped}" unless skipped.blank? )
+
+        else
+          super
+        end
+      end
+    end
+  end
+end
diff --git a/lib/arel/engines/sql/compilers/sqlite_compiler.rb b/lib/arel/engines/sql/compilers/sqlite_compiler.rb
new file mode 100644
index 0000000000..c2f78cd235
--- /dev/null
+++ b/lib/arel/engines/sql/compilers/sqlite_compiler.rb
@@ -0,0 +1,9 @@
+module Arel
+  module SqlCompiler
+    class SQLiteCompiler < GenericCompiler
+      def locked
+        nil
+      end
+    end
+  end
+end