You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: snapshot_item_test.rb
    path of file: ./repos/active_snapshot/test/models
    
    the code of the file until where you have to start completion: require "test_helper"

class SnapshotItemTest < ActiveSupport::TestCase

  def setup
    @snapshot_klass = ActiveSnapshot::Snapshot
    @snapshot_item_klass = ActiveSnapshot::SnapshotItem
  end

  def teardown
  end

  def test_relationships
    instance = @snapshot_item_klass.new

    assert instance.snapshot
    
  def test_validations
    instance = @snapshot_item_klass.new

    instance.valid?

    [:item_id, :item_type, :snapshot_id].each do |attr|
      assert_equal ["can't be blank"], instance.errors[attr] ### presence error
    end

    shared_post = DATA[:shared_post]
    snapshot = shared_post.snapshots.first

    instance = @snapshot_item_klass.new(item: snapshot.item, snapshot: snapshot)

    assert_not instance.valid?

    assert_equal ["has already been taken"], instance.errors[:item_id] ### uniq error
  end