validates_presence_of :comment

That checks for the field name comment, like in the code snippet below. This will produce the input tag using rails.

<%= text_field "comment", "comment" %>

The other section to make it all run is the

<%= error_messages_for 'model_name' %>

Place this line of code at the top of your form, and replace “model_name” with the model name your application refers too. It tells the application to look at the validation rules you set up. Once you have those in place, you should not be able to submit the form with a blank input.

A few other predefined rules built in to Ruby on Rails are:

Field should not be empty

Code: validates_presence_of
Example: validates_presence_of :user_name, :password

Field should Numerical

Code: validates_numericality_of
Example: validates_numericality_of :value

Field should not be Unique

Code:validates_uniqueness_of
Example: validates_uniqueness_of :user_name

Field Need to be of size X

Code: validates_length_of
Example: validates_length_of :state_code, :is=>2

Field size Need to be of between X to Y

Code: validates_length_of
Example: validates_uniqueness_of :user_name

To learn more about validation and to read up on the rest of the Rails API, read this

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