Rails: How to handle "Attribute was supposed to be a Array, but was a String" error?

From the fine manual:

serialize(attr_name, class_name = Object)
[...] The serialization is done through YAML.

So the column should contain a YAMLized version of your image_urls but '["image1.jpg", "image2.jpg"]' is not a YAML array. If you want to muck around with the raw serialized data then you should use something like

["image1.jpg", "image2.jpg"].to_yaml
# ---------------------------^^^^^^^

to generate the string.

Or better, stop using serialize altogether in favor of a separate table.

In Rails 4 it worked for me without any .to_yaml, just added it to strong params as image_urls: []