Conditional model validations based on a role - ruby-on-rails

Question

I have roles for users implemented with an enum in the user model:
enum role: [:staff, :clinician]
I have a University model with the User belongs_to :university and a University model with has_many :users.
The way that my app will work is that "staff" will belong to a university, but "clinicians" are private practice and therefore do not need to belong to a university and need not select one during signup.
I have my signup form set up to hide the university field if the user selects Clinician, but I want to make sure that my validations are set up to require that any user who selects staff on signup must also select a university and that any user who selects clinician on signup fails validation if they select a university.
Here's the role section of the user signup form:
<%= f.label :role %>
<%= f.select(:role, User.roles.keys.map {|role| [role.titleize,role]}, :include_blank => "Please Select", id: 'user_role') %>
<%= content_tag :div, class: 'js-dependent-fields', data: { 'select-id': 'user_role', 'option-value': 'staff'} do %>
<%= f.label :university_id%>
<%= collection_select( :user, :university_id, University.all, :id, :name, prompt: true) %>

Answer 1

It requires a bit more extra setup but I think pays off in flexibility over time:
Try Single Table Inheritance combined with your enum roles. You'll be able to more easily define separate callbacks, validations, scopes, and associations for your different roles, while inheriting the ones you want them to share in common. For example, you could just make it so only Staff belongs_to :university, and Clinician does not.
# Stripped down schema
create_table "universities", force: :cascade do |t|
t.datetime "created_at", null: false
t.datetime "updated_at", null: false
end
create_table "users", force: :cascade do |t|
t.datetime "created_at", null: false
t.datetime "updated_at", null: false
t.bigint "university_id"
t.integer "role"
t.index ["university_id"], name: "index_users_on_university_id"
end
# Models
class University < ApplicationRecord
has_many :staffs
end
class User < ApplicationRecord
self.inheritance_column = :role
enum role: { Staff: 0, Clinician: 1 }
end
class Clinician < User
end
class Staff < User
belongs_to :university
end
Staff.first.university # => returns instance of University
Clinician.first.university # => raises NoMethodError
University.first.staffs # => returns collection of Staff objects
University.first.clinicians # => raises NoMethodError
Note that there is no type column. It's been overridden by the role:integer column used for the enum by setting self.inheritance_column = :role. You can interact with the enum roles with the string/symbol representation ("Staff", Staff.new, User.first.Staff?, User.first.Staff!, User.new(role: "Staff") and ActiveRecord takes care of converting that string to the right integer for the database queries.
For example, here's the query for User.where(role: "Staff")
SELECT "users".* FROM "users" WHERE "users"."role" = 0
Staff.all returns the same result but the wording of the query is slightly different
SELECT "users".* FROM "users" WHERE "users"."role" IN (0)
See this question for more detail: Same Model with different columns Rails

Answer 2

You can give a condition to the validates call in your User.rb model:
validates :university, presence: true, if: lambda { self.role.to_s == 'staff' }
# watch out for symbol vs. string in your self.role array
And I think (never done it but I guess that would work) you can do this for the :clinician role:
validates :university, presence: false, if: lambda { self.role.to_s == 'clinician' }