Improve change_column

This change allows to change the definition of the last column in a
table (it's not possible to remove all columns from a table) and
speedups the migration as only one alter statement is necessary for each
change_column.

lib/lhm/migrator.rb

@@ -66,8 +66,7 @@ def add_column(name, definition)
     # @param [String] name Name of the column to change
     # @param [String] definition Valid SQL column definition
     def change_column(name, definition)
-      remove_column(name)
-      add_column(name, definition)
+      ddl("alter table `%s` modify column `%s` %s" % [@name, name, definition])
     end
 
     # Remove a column from a table

spec/fixtures/small_table.ddl

@@ -0,0 +1,4 @@
+CREATE TABLE `small_table` (
+  `id` INT(11),
+  PRIMARY KEY (`id`)
+) ENGINE=InnoDB DEFAULT CHARSET=utf8

spec/integration/lhm_spec.rb

@@ -119,6 +119,22 @@
       end
     end
 
+    it "should change the last column in a table" do
+      table_create(:small_table)
+
+      Lhm.change_table(:small_table) do |t|
+        t.change_column(:id, "int(5)")
+      end
+
+      slave do
+        table_read(:small_table).columns["id"].must_equal({
+          :type => "int(5)",
+          :is_nullable => "NO",
+          :column_default => "0"
+        })
+      end
+    end
+
     describe "parallel" do
       it "should perserve inserts during migration" do
         50.times { |n| execute("insert into users set reference = '#{ n }'") }

spec/unit/migrator_spec.rb

@@ -66,11 +66,10 @@
     end
 
     it "should change a column" do
-      @creator.change_column("logins", "INT(255)")
+      @creator.change_column("logins", "INT(11)")
 
       @creator.statements.must_equal([
-        "alter table `lhmn_alt` drop `logins`",
-        "alter table `lhmn_alt` add column `logins` INT(255)"
+        "alter table `lhmn_alt` modify column `logins` INT(11)"
       ])
     end
   end