add submission_versions table and index

This change is in anticipation of adding a more data inclusive grade
history API. Adds user_id and assignment_id to index as well.

Test Plan: n/a

refs CNVS-2802

Change-Id: Ib9f305612bbc48a6423ed955cd591d6de0658eb0
Reviewed-on: https://gerrit.instructure.com/19505
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jacob Fugal <jacob@instructure.com>
Product-Review: Duane Johnson <duane@instructure.com>
QA-Review: Duane Johnson <duane@instructure.com>

db/migrate/20130411031858_add_table_submission_versions.rb

@@ -0,0 +1,29 @@
+class AddTableSubmissionVersions < ActiveRecord::Migration
+  tag :predeploy
+
+  def self.up
+    create_table :submission_versions do |t|
+      t.integer  "context_id", :limit => 8
+      t.string   "context_type"
+      t.integer  "version_id", :limit => 8
+      t.integer  "user_id", :limit => 8
+      t.integer  "assignment_id", :limit => 8
+    end
+    case connection.adapter_name
+    when 'PostgreSQL'
+      connection.execute(
+        "CREATE INDEX index_submission_versions " +
+        "ON submission_versions (context_id, version_id, user_id, assignment_id) " +
+        "WHERE context_type = 'Course'")
+    else
+      add_index :submission_versions,
+        [:context_id, :context_type, :version_id, :user_id, :assignment_id],
+        :name => 'index_submission_versions',
+        :unique => true
+    end
+  end
+
+  def self.down
+    drop_table :submission_versions
+  end
+end