[Rails] Validates attributes that aren't even being changed

I have a form where I can only update the user's first name, last name
and email address.

I have this in the user model file, user.rb

validates :username,
:presence => true,
:uniqueness => true,
:on => :update

validates :password,
:presence => true,
:length => { :minimum => 6, :maximum => 20},
:confirmation => true,
:on => :update

But everytime I try to change the user's first name, I get the error
messages that the username is already taken and such. I'm not even
trying to update the username. And there is NOT EVEN A FIELD for the
username in the form.

When I try to update the user's first name in the console, I get the
same errors.

How can I make Rails NOT run that validation when I'm trying to update
other fields?

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