Customize error message with simple_form

1. You can declare the content of the error message in your model:

   validates_length_of :name, :minimum => 5, :message => "blah blah blah"
   

2. You can set id or class for your error tag:

   <%= f.input :name, :error_html => { :id => "name_error"} %> 
   

   Then you can use CSS for the styling.

3. And you can use

   <%= f.error :name, :id => "name_error" %>
   

   and you'll get

   <span class="error" id="name_error">is too short (minimum is 5 characters)</span>
   

I dont know if it is any different for simple_form gem.

For content of error messages to be changed, you can use the :message attribute in the model.

class User < ActiveRecord::Base
  validates :email, {:presence => true, :message => "is not filled up."}
end

Now the validation message will be Email is not filled up. If you want the field name also to be changed(Email to E-mail address something like that ), the approach now is to define it in locales.rb file like this

# config/locales/en.yml
en:
  activerecord:
    attributes:
      user:
        email: "E-mail address"

See link for details on locales. Another approach is to define in the model, humanized attributes like this:

class User < ActiveRecord::Base
  validates :email, {:presence => true, :message => "is not filled up."}
  HUMANIZED_ATTRIBUTES = {
    :email => "E-mail address",
    ...(other fields and their humanized names)
    ...
  }

  def self.human_attribute_name(attr, options={})
    HUMANIZED_ATTRIBUTES[attr.to_sym] || super
  end

end

For customizing style of validation message we will have to edit the style for #errorExplanation and .fieldWithErrors,in the scaffold.css stylesheet.

There is another solution explained here that wasn't mentioned in the answers. You can directly override the error messages from the views, in the form itself. For example:

<%= f.input :last_name,
               placeholder: 'last_name',
               error: 'This is a custom error message',
               required: true,
               class: 'form-field',
               autofocus: true,
               input_html: { autocomplete: "last_name" } %>

It is however not advised as it is not DRY, you would need to override it in every field.