Explanation of problem:

Rails serialized column indeed allow us to store complex data types such as array, but they do so in YAML format by default. What you do is pass a text as group[cords][] param. So, what you should get in your params is {group: {cords: [ "[[1,2],[3,4],[5.5,6]]" ]}}. Take notice, that cords is array of one element, which is string you pass to it: "[[1,2],[3,4],[5.5,6]]"

That array gets serialized in compliance with YAML scalar syntax: "---\n- \"[[1,2],[3,4],[5.5,6]]\"\n". Let's look at it up close, first substituting newlines:

---
- "[[1,2],[3,4],[5.5,6]]"
#here also newline

Three hyphens indicate start of YAML document.

Array is packed like this:

- "first_element"
- "second_element"
- "third_element"

Is equal to ["first_element", "second_element", "third_element"]

Backslash before quotation marks just escape those in SQL denoting that quotes are part of the string.

So, as you can see, rails do just what it is supposed / you told it to do: it gets single string as one element of array and it stores one element of array, containing that string.

Solution:

What you should do if you still want to get user input as array is transform that manually.

• Drop the [] in text_field: f.text_field :cords, name: "group[cords]"
• In controller, transform that text into ruby array, for example like this:

def cords_to_a
  # get the params[:group][:cords], if it's empty use empty array string
  cords = params.dig(:group, :cords).presence || "[]"
  # parse it, possibly you also want to handle parse exceptions
  # and transform parsed array as something else
  # also, user will be able to save any json object
  # to your serialized field, and it will be parsed
  # accordingly: either to array or to hash
  JSON.parse cords
end

def group_params
  params.require(:group).permit(:name, :members).merge({cords: cords_to_a})
end

and after that use Group.new(group_params)

Take notice: you still will get a "strange" string in your INSERT statement, because that will contain YAML-encoded array: essentially, a string.

Further reading:

• YAML syntax
• Rails serialized fields source with doc