The Simple Form gem gives us some advanced features built on top of Rails’ form builder. One such feature is the ability to infer the maxlength attribute by reading the validations on an ActiveRecord model. This can be set by opening the simple form initializer and changing b.optional :maxlength to b.use :maxlength. This file is well documented with comments.

# config/initializers/simple_form.rb

SimpleForm.setup do |config|
  config.wrappers :default, :class: input,
    hint_class: :field_with_hint, error_class: :field_with_errors do

    ## Optional extensions
    # They are disabled unless you pass `f.input EXTENSION_NAME => true`
    # to the input. If so, they will retrieve the values from the model
    # if any exists. If you want to enable any of those
    # extensions by default, you can change `b.optional` to `b.use`.

    # Calculates maxlength from length validations for string inputs
    b.optional :maxlength
  end
end