ROR: Search by Username and display their posts - ruby-on-rails

Question

I am learning Ruby on Rails and have a search form set up and its working. On the pins index view I can search for pins(posts) by their title. However if I wanted to search by Username which is not in the pins table and display the results on the Pins index page how would I do this? How do I access an attribute from a different table? (Sorry for the newbie attempt at explaining my issue)
Pins controller
def index
#pins = Pin.search(params[:term])
end
Pin Model
def self.search(term)
if term
where('description LIKE ?', "%#{term}%")
else
order('id DESC')
end
end
_search.html.erb
<%= form_tag(pins_path, method: :get) do %>
<%= text_field_tag :term, params[:term] %>
<%= submit_tag 'Search', description: nil %>
<% end %>

Answer 1

Assuming you have set up something like
class User < ApplicationRecord
has_many :pins
# the username is stored in the attribute 'username'
end
class Pin < ApplicationRecord
belongs_to :user
end
you may do the following
# PinsController
def index
terms = params[:term]
username = params[:username]
#pins = Pin
#pins = #pins.where("description LIKE '%?%'", term) if term
#pins = #pins.includes(:user).where("users.username LIKE '%?%'", username) if username
# you may want to sort by id anyway
#pins = #pins.order('id DESC')
end
Note that I put the code straight to the controller for brevity. You may refactor this to use your search method in pin model.
# _search.html.erb
<%= form_tag(pins_path, method: :get) do %>
<%= text_field_tag :term, params[:term] %>
<%= text_field_tag :username, params[:username] %>
<%= submit_tag 'Search', description: nil %>
<% end %>
In case you want to do some more searching and filtering you may have a look at the ransack gem although I think you're going the right path in trying to figure this out yourself.
Although those railscasts episodes are from the past I think they are applicable to the current rails versions. Anyway one can get the point from them
http://railscasts.com/episodes/37-simple-search-form
http://railscasts.com/episodes/111-advanced-search-form
http://railscasts.com/episodes/240-search-sort-paginate-with-ajax
Another good resource is gorails.com (not affiliate in any way!!). I can highly recommend them as a resource for learning

Answer 2

Provided you have an association between User and Pin
class User
has_many :pins
end
class Pin
belongs_to :user
end
You can join :user from Pin and set conditions on the association:
Pin.joins(:user).where('users.username ? AND awesome = ?', 'Max', true)
# or the preferred method
Pin.joins(:user).where(user: { username: 'Max', awesome: true })
Note that we use users.username and not user.username when writing a SQL string you're specifying the table name - not the association.
To search for pins based on the username you could do:
def self.by_username(term)
joins(:user).where('users.username LIKE ?', "%#{term}%")
end