Allow precision option for postgresql datetimes

This patch addresses the difficulty of retrieving datetime fields. By default, the database holds a higher precision than the time as a String.

This issue is discussed at length at the following links:
- [#3519](https://github.com/rails/rails/issues/3519)
- [#3520](https://github.com/rails/rails/issues/3520)

Also, kudos to @mattscilipoti

activerecord/CHANGELOG.md

@@ -1,5 +1,17 @@
 ## Rails 4.0.0 (unreleased) ##
 
+*   Added support for specifying the precision of a timestamp in the postgresql
+    adapter. So, instead of having to incorrectly specify the precision using the
+    `:limit` option, you may use `:precision`, as intended. For example, in a migration:
+
+        def change
+          create_table :foobars do |t|
+            t.timestamps :precision => 0
+          end
+        end
+
+    *Tony Schneider*
+
 *   Allow ActiveRecord::Relation#pluck to accept multiple columns. Returns an
     array of arrays containing the typecasted values:
 

activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb

@@ -187,6 +187,7 @@ module ActiveRecord
         case sql_type
         when /^bigint/i;    8
         when /^smallint/i;  2
+        when /^timestamp/i; nil
         else super
         end
       end
@@ -201,6 +202,8 @@ module ActiveRecord
       def extract_precision(sql_type)
         if sql_type == 'money'
           self.class.money_precision
+        elsif sql_type =~ /timestamp/i
+          $1.to_i if sql_type =~ /\((\d+)\)/
         else
           super
         end
@@ -1285,6 +1288,13 @@ module ActiveRecord
             when 5..8; 'bigint'
             else raise(ActiveRecordError, "No integer type has byte size #{limit}. Use a numeric with precision 0 instead.")
           end
+        when 'datetime'
+          return super unless precision
+
+          case precision
+            when 0..6; "timestamp(#{precision})"
+            else raise(ActiveRecordError, "No timestamp type has precision of #{precision}. The allowed range of precision is from 0 to 6")
+          end
         else
           super
         end

activerecord/test/cases/adapters/postgresql/timestamp_test.rb

@@ -27,4 +27,69 @@ class TimestampTest < ActiveRecord::TestCase
     d = Developer.create!(:name => 'aaron', :updated_at => -1.0 / 0.0)
     assert_equal(-1.0 / 0.0, d.updated_at)
   end
+
+  def test_default_datetime_precision
+    ActiveRecord::Base.connection.create_table(:foos)
+    ActiveRecord::Base.connection.add_column :foos, :created_at, :datetime
+    ActiveRecord::Base.connection.add_column :foos, :updated_at, :datetime
+    assert_nil activerecord_column_option('foos', 'created_at', 'precision')
+  end
+
+  def test_timestamp_data_type_with_precision
+    ActiveRecord::Base.connection.create_table(:foos)
+    ActiveRecord::Base.connection.add_column :foos, :created_at, :datetime, :precision => 0
+    ActiveRecord::Base.connection.add_column :foos, :updated_at, :datetime, :precision => 5
+    assert_equal 0, activerecord_column_option('foos', 'created_at', 'precision')
+    assert_equal 5, activerecord_column_option('foos', 'updated_at', 'precision')
+  end
+
+  def test_timestamps_helper_with_custom_precision
+    ActiveRecord::Base.connection.create_table(:foos) do |t|
+      t.timestamps :precision => 4
+    end
+    assert_equal 4, activerecord_column_option('foos', 'created_at', 'precision')
+    assert_equal 4, activerecord_column_option('foos', 'updated_at', 'precision')
+  end
+
+  def test_passing_precision_to_timestamp_does_not_set_limit
+    ActiveRecord::Base.connection.create_table(:foos) do |t|
+      t.timestamps :precision => 4
+    end
+    assert_nil activerecord_column_option("foos", "created_at", "limit")
+    assert_nil activerecord_column_option("foos", "updated_at", "limit")
+  end
+
+  def test_invalid_timestamp_precision_raises_error
+    assert_raises ActiveRecord::ActiveRecordError do
+      ActiveRecord::Base.connection.create_table(:foos) do |t|
+        t.timestamps :precision => 7
+      end
+    end
+  end
+
+  def test_postgres_agrees_with_activerecord_about_precision
+    ActiveRecord::Base.connection.create_table(:foos) do |t|
+      t.timestamps :precision => 4
+    end
+    assert_equal '4', pg_datetime_precision('foos', 'created_at')
+    assert_equal '4', pg_datetime_precision('foos', 'updated_at')
+  end
+
+  private
+
+    def pg_datetime_precision(table_name, column_name)
+      results = ActiveRecord::Base.connection.execute("SELECT column_name, datetime_precision FROM information_schema.columns WHERE table_name ='#{table_name}'")
+      result = results.find do |result_hash|
+        result_hash["column_name"] == column_name
+      end
+      result && result["datetime_precision"]
+    end
+
+    def activerecord_column_option(tablename, column_name, option)
+      result = ActiveRecord::Base.connection.columns(tablename).find do |column|
+        column.name == column_name
+      end
+      result && result.send(option)
+    end
+
 end