gistfile1.rb

# /app/models/product.rb
validates :title, :uniqueness   => true, 
                  :length       => {:minimum => 10,
                                    :message => 'Title is too short. Must be at least 10 characters'}

# /test/unit/product_test.rb
test "product title must be at least 10 characters long" do
    product = Product.new(:description => "yyy",
                          :price       => 1,
                          :image_url   => "fred.gif")
    product.title = "Incorrect"
    assert product.invalid?
    assert_equal 'Title is too short. Must be at least 10 characters', product.errors[:title].join('; ')

    product.title "And this"
    assert product.invalid?
    assert_equal 'Title is too short. Must be at least 10 characters', product.errors[:title].join('; ')

    product.title "At least 10 characters long is correct"
    assert product.valid?
end

--- OR ---

# /app/models/product.rb
validates :title, :uniqueness   => true, 
                  :length       => {:minimum => 10,
                                    :message => :too_short}

# /test/unit/product_test.rb
test "product title must be at least 10 characters long" do
    product = Product.new(:description => "yyy",
                          :price       => 1,
                          :image_url   => "fred.gif")
    product.title = "Incorrect"
    assert product.invalid?
    assert_equal :too_short, product.errors[:title].join('; ')

    product.title "And this"
    assert product.invalid?
    assert_equal :too_short, product.errors[:title].join('; ')

    product.title "At least 10 characters long is correct"
    assert product.valid?
end