Associating with Ruby on Rails
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What is Association
all about?
Accessing info from Database B via Database A.
And vice versa.
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A.B.attribute
and/or
B.A.attribute
Types of Association
One-to-One:
The simplest form of Association, where one instance of a Resource "has one" of the other, and that instance of the other resource "belongs to" one of the first
One-to-Many:
Where one instance of a Resource "has many" of the other, and those instances of the other resource only "belong(s) to" that instance of the first.
Many-to-Many:
The hard one! Where instances of a Resource can "have (has) many" instances of the other Resource, and many instances of that Resource can "have (has) many" right back. But this type of Association will require a 3rd DB: the Join Table!
One-to-one Association
Let's create a one-to-one association, between two databases: Driver & Vehicle
$ rails g scaffold Driver name:string
$ rails g scaffold Vehicle make:string model:string driver_id:integer
Notice the driver_id attribute of the Vehicle!
This is called the "Foreign Key" (FK). It relates to the ID number of a Driver in its own database.
Warning! If you're using any other name for your first DB than "Driver", make sure the _id attribute in Vehicle matches!
(e.g., pilot_id, owner_id, etc.)
Association Set-up, steps 2 & 3
The next two steps to setting up an association are defining the relationship in the respective models.
driver.rb
vehicle.rb
class Vehicle < ActiveRecord::Base
belongs_to :driver
end
class Driver < ActiveRecord::Base
has_one :vehicle
end
Drivers have one Vehicle
Vehicles belong to one Driver
And now we're associated!
That's really all it takes to do a one-to-one association!
The trick now is to make the calls between the databases...
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who's driving this thing anyway?
In our Vehicle _form.html.erb, we want to pull in a list of our Drivers, to assign their IDs to the new (or updated) Vehicle.
First, we need to pull in the Drivers data in the vehicle_controller.rb:
def new
@vehicle = Vehicle.new
@drivers = Driver.all
end
def edit
@drivers = Driver.all
end
who's driving this thing anyway?
Now we can create a dropdown menu to select Drivers from, by name!
In vehicles/_form.html.erb:
<div class="field">
<%= f.label :make %><br>
<%= f.text_field :make %>
</div>
<div class="field">
<%= f.label :model %><br>
<%= f.text_field :model %>
</div>
<div class="field">
<%= f.label :driver_id %><br>
<!-- The line below is our new bit of code: -->
<%= f.select(:driver_id, @drivers.collect {|d| [ d.name, d.id ] }, {:include_blank =>
'Please select a item'}) %>
</div>
<div class="actions">
<%= f.submit %>
</div>
choose your driver
Now we have a nice dropdown selection tool:
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We see the name, but it's the ID that's passed!
show your associations!
Let's edit the display pages (index & show) of the two resources to visualize the association.
Start with the show pages:
vehicles/show.html.erb
drivers/show.html.erb
<p id="notice"><%= notice %></p>
<p>
<strong>Make:</strong>
<%= @vehicle.make %>
</p>
<p>
<strong>Model:</strong>
<%= @vehicle.model %>
</p>
<!-- We'll display the Driver's name instead of the ID-->
<!-- First, set up a safeguard against nil -->
<% if @vehicle.driver != nil %>
<p>
<strong>Driver:</strong>
<!-- display the Driver's name thru assoc! -->
<%= @vehicle.driver.name %>
</p>
<% end %>
<%= link_to 'Edit', edit_vehicle_path(@vehicle) %> |
<%= link_to 'Back', vehicles_path %>
<p id="notice"><%= notice %></p>
<p>
<strong>Name:</strong>
<%= @driver.name %>
</p>
<p>
<strong>City:</strong>
<%= @driver.city %>
</p>
<!-- We'll display the Driver's Vehicle -->
<!-- First, set up a safeguard against nil -->
<% if @driver.vehicle != nil %>
<p>
<strong>Vehicle:</strong>
<!-- display the Vehicle's make & model
thru assoc! -->
<%= "#{@driver.vehicle.make}
#{@driver.vehicle.model}" %>
</p>
<% end %>
<%= link_to 'Edit', edit_driver_path(@driver) %> |
<%= link_to 'Back', drivers_path %>
show your associations!
Now we can edit the index pages:
(I'm showing just the tables from these pages)
vehicles/index.html.erb
drivers/index.html.erb
<table>
<thead>
<tr>
<th>Make</th>
<th>Model</th>
<!-- We'll display the Driver's name instead of the ID-->
<th>Driver</th>
<th colspan="3"></th>
</tr>
</thead>
<tbody>
<% @vehicles.each do |vehicle| %>
<tr>
<td><%= vehicle.make %></td>
<td><%= vehicle.model %></td>
<td>
<!-- set up a safeguard against nil -->
<% if vehicle.driver != nil %>
<!-- display the Driver's name thru association! -->
<%= vehicle.driver.name %>
<% end %>
</td>
<td><%= link_to 'Show', vehicle %></td>
<td><%= link_to 'Edit', edit_vehicle_path(vehicle) %></td>
<td><%= link_to 'Destroy', vehicle, method: :delete,
data: { confirm: 'Are you sure?' } %></td>
</tr>
<% end %>
</tbody>
</table>
<table>
<thead>
<tr>
<th>Name</th>
<th>City</th>
<!-- We'll display the Driver's Vehicle -->
<th>Vehicle</th>
<th colspan="3"></th>
</tr>
</thead>
<tbody>
<% @drivers.each do |driver| %>
<tr>
<td><%= driver.name %></td>
<td><%= driver.city %></td>
<td>
<!-- set up a safeguard against nil -->
<% if driver.vehicle != nil %>
<!-- display the Vehicle's make & model thru assoc! -->
<%= "#{driver.vehicle.make} #{driver.vehicle.model}" %>
<% end %>
</td>
<td><%= link_to 'Show', driver %></td>
<td><%= link_to 'Edit', edit_driver_path(driver) %></td>
<td><%= link_to 'Destroy', driver, method: :delete,
data: { confirm: 'Are you sure?' } %></td>
</tr>
<% end %>
</tbody>
</table>
</table>
I can see your associations!
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One-to-many association
A One-to-Many Association is very similar to the One-to-One.
There's a just a slight change of verbiage.
But we'll start off the same.
Let's associate States with Cities...
We still have one Resource taking the Foreign Key (ID) of the other:
state_id
$ rails g scaffold State name:string
$ rails g scaffold City name:string state_id:integer
"One-to-many" set-up, Steps 2 & 3
Once again we will define the relationship
between the resources in the respective models.
state.rb
city.rb
class City < ActiveRecord::Base
belongs_to :state
end
class State < ActiveRecord::Base
has_many :cities
end
State has many Cities
(instead of has_one)
A City belongs to one State
(same as 1-to-1 assoc.)
Selecting States
Let's set up the State selection in the City
form just as we did with Driver/Vehicle.
cities/_form.html.erb
cities_controller.rb
def new
@city = City.new
@states = State.all
end
def edit
@states = State.all
end
<div class="field">
<%= f.label :name %><br>
<%= f.text_field :name %>
</div>
<div class="field">
<%= f.label :state_id %><br>
<!-- Copy the select tag from the
Vehicles form and adjust it to
fit our needs here... -->
<%= f.select(:state_id,
@states.collect {|s| [ s.name, s.id ] },
{:include_blank => 'Please select
an item'}) %>
</div>
<div class="actions">
<%= f.submit %>
</div>
Choose Your state
Voila! A nice dropdown menu to choose our city's state!
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Display your States!
Showing the City, State association will be about the same as the Vehicle/Driver association.
cities/index.html.erb
cities/show.html.erb
<table>
<thead>
<tr>
<th>Name</th>
<th>State</th>
<th colspan="3"></th>
</tr>
</thead>
<tbody>
<% @cities.each do |city| %>
<tr>
<td><%= city.name %></td>
<!-- we only need to switch this line
from city.name_id to: -->
<td><%= city.state.name %></td>
<td><%= link_to 'Show', city %></td>
<td><%= link_to 'Edit', edit_city_path(city) %></td>
<td><%= link_to 'Destroy', city, method: :delete, data:
{ confirm: 'Are you sure?' } %></td>
</tr>
<% end %>
</tbody>
</table>
<p id="notice"><%= notice %></p>
<!-- On this page, all we have to do is
display the city name and the state name,
preferably side-by-side, with a comma between
as is the preferred style -->
<p>
<strong>Welcome To...</strong>
<%= @city.name %>, <%= @city.state.name %>
</p>
<%= link_to 'Edit', edit_city_path(@city) %> |
<%= link_to 'Back', cities_path %>
I'm from the great state of...
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localhost:3000/cities
localhost:3000/cities/1
What cities are in that state?
Showing the State/Cities association will be a bit trickier.
The cities associated with a State are collect in an Array.
states/show.html.erb
<p id="notice"><%= notice %></p>
<p>
<strong>The Great State of... </strong>
<%= @state.name %>
</p>
<p>
<strong>Major Cities:</strong>
<ul>
<!-- We'll loop through the array of cities
and present them as an unordered list -->
<% @state.cities.each do |city| %>
<li><%= city.name %></li>
<% end %>
</ul>
</p>
<%= link_to 'Edit', edit_state_path(@state) %> |
<%= link_to 'Back', states_path %>
Cities in our state
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localhost:3000/states/1
Displaying cities in the state INdex
Once again, we'll have to loop through an array of cities, but this time let's present them in a one-line list...
states/index.html.erb
<table>
<thead>
<tr>
<th>Name</th>
<!-- We add a header for the cities column -->
<th>Cities</th>
<th colspan="3"></th>
</tr>
</thead>
<tbody>
<% @states.each do |state| %>
<tr>
<td><%= state.name %></td>
<!-- We add a table cell that will display the cities within the state-->
<td>
<!-- In here we'll loop through the array of cities, printing them out with a comma between
(the last city in the array will not have a comma after it) -->
<% state.cities.each_with_index do |city, index| %>
<% if index < (state.cities.length - 1) %>
<%= city.name %>,
<% else %>
<%= city.name %>
<% end %>
<% end %>
</td>
<td><%= link_to 'Show', state %></td>
<td><%= link_to 'Edit', edit_state_path(state) %></td>
<td><%= link_to 'Destroy', state, method: :delete, data: { confirm: 'Are you sure?' } %></td>
</tr>
<% end %>
</tbody>
</table>
Cities in our State, Index-style
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localhost:3000/states
Many-to-many Association
The Many-to-Many association is the trickiest of the bunch.
Neither of the two main scaffolds will include an FK attribute.
A third scaffold will be needed, and it will only have FK attributes.
This is the Join Table.
We'll explore Many-to-Many using a Doctor/Patient relationship, with Appointment being the JoinTable.
$ rails g scaffold Doctor name:string
$ rails g scaffold Patient name:string
$ rails g scaffold Appointment doctor_id:integer patient_id:integer
Setting up the Models
In the Many-to-Many case, both Doctors and Patients have many appointments.
But they also have many of each other, but only through Appointments.
Finally, Appointments belong to both Doctors & Patients.
We'll start with...
appointment.rb
class Appointment < ActiveRecord::Base
belongs_to :doctor
belongs_to :patient
end
Setting up the models
Now let's see the other two models, and how exactly this through relationship takes shape.
doctor.rb
patient.rb
class Patient < ActiveRecord::Base
has_many :appointments
has_many :doctors, :through => :appointments
end
class Doctor < ActiveRecord::Base
has_many :appointments
has_many :patients, :through => :appointments
end
Wait...that's it?
As before, setting up the Association is kinda easy. Include a FK here, write some code in a model, and PRESTO!, you got yourself some Associations.
The tricky park comes when you want to display the results of those Associations.
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Get yourself some data
First, let create several Doctors and Patients, so we have some data to work with.
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localhost:3000/doctors
localhost:3000/patients
Making Appointments
Before we start creating appointments, to associate our Doctors & Patients, let's set it up so that we choose from Doctor & Patient names, rather than having to supply ID numbers.
def new
@appointment = Appointment.new
@doctors = Doctor.all
@patients = Patient.all
end
def edit
@doctors = Doctor.all
@patients = Patient.all
end
<div class="field">
<%= f.label :doctor_id %><br>
<!-- Remember the select tags we used in the
association examples? Let's use that again,
twice this time! - adjusting the wording to
fit our purposes here. -->
<%= f.select(:doctor_id, @doctors.collect
{|d| [ d.name, d.id ] }, {:include_blank =>
'Please select a item'}) %>
</div>
<div class="field">
<%= f.label :patient_id %><br>
<%= f.select(:patient_id, @patients.collect
{|p| [ p.name, p.id ] }, {:include_blank =>
'Please select a item'}) %>
</div>
<div class="actions">
<%= f.submit %>
</div>
appointments_controller.rb
appointments/_form.html.erb
making appointment is fun!
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Displaying Appointments
Configuring the Appointments pages to show Doctor and Patients names is the easiest of the three.
appointments/index.html.erb
<table>
<thead>
<tr>
<th>Doctor</th>
<th>Patient</th>
<th colspan="3"></th>
</tr>
</thead>
<tbody>
<% @appointments.each do |appointment| %>
<tr>
<td>
<%= appointment.doctor.name %>
</td>
<td><%= appointment.patient.name %></td>
<td><%= link_to 'Show', appointment %></td>
<td><%= link_to 'Edit', edit_appointment_path(appointment) %></td>
<td><%= link_to 'Destroy', appointment, method: :delete, data: { confirm: 'Are you sure?' } %></td>
</tr>
<% end %>
</tbody>
</table>
All the appointments!
It might be nice to add appointment date and time attribute(s) to this Resource. Just sayin'.
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Doctors & their Patients: The code
As taken separately, the Doctor/Patients and Patient/Doctors relationships are essentially One-to-Many relationships. So we'll be needing to loop through an array.
<table>
<thead>
<tr>
<th>Name</th>
<th>Patients</th>
<th colspan="3"></th>
</tr>
</thead>
<tbody>
<% @doctors.each do |doctor| %>
<tr>
<td><%= doctor.name %></td>
<td>
<% doctor.patients.each_with_index do |pat, index| %>
<% if index < (doctor.patients.length - 1) %>
<%= pat.name %>,
<% else %>
<%= " #{pat.name}" %>
<% end %>
<% end %>
</td>
<td><%= link_to 'Show', doctor %></td>
<td><%= link_to 'Edit', edit_doctor_path(doctor) %></td>
<td><%= link_to 'Destroy', doctor, method: :delete, data:
{ confirm: 'Are you sure?' } %></td>
</tr>
<% end %>
</tbody>
</table>
<p id="notice"><%= notice %></p>
<p>
<strong>Name:</strong>
<%= @doctor.name %>
</p>
<p>
<strong>Patients:</strong>
<ul>
<!-- We'll loop through the array of patients
and present them as an unordered list -->
<% @doctor.patients.each do |patient| %>
<li><%= patient.name %></li>
<% end %>
</ul>
</p>
<%= link_to 'Edit', edit_doctor_path(@doctor) %> |
<%= link_to 'Back', doctors_path %>
doctors/index.html.erb
doctors/show.html.erb
Doctors & their Patients: In the Browser
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localhost:3000/doctors
localhost:3000/doctors/1
Patients & their Doctors: The code
<table>
<thead>
<tr>
<th>Name</th>
<th>Doctors</th>
<th colspan="3"></th>
</tr>
</thead>
<tbody>
<% @patients.each do |patient| %>
<tr>
<td><%= patient.name %></td>
<td>
<% patient.doctors.each_with_index do |doc, index| %>
<% if index < (patient.doctors.length - 1) %>
<%= doc.name %>,
<% else %>
<%= " #{doc.name}" %>
<% end %>
<% end %>
</td>
<td><%= link_to 'Show', patient %></td>
<td><%= link_to 'Edit', edit_patient_path(patient) %></td>
<td><%= link_to 'Destroy', patient, method: :delete, data:
{ confirm: 'Are you sure?' } %></td>
</tr>
<% end %>
</tbody>
</table>
<p id="notice"><%= notice %></p>
<p>
<strong>Name:</strong>
<%= @patient.name %>
</p>
<p>
<strong>Doctors:</strong>
<ul>
<!-- We'll loop through the array of doctors
and present them as an unordered list -->
<% @patient.doctors.each do |doctor| %>
<li><%= doctor.name %></li>
<% end %>
</ul>
</p>
<%= link_to 'Edit', edit_patient_path(@patient) %> |
<%= link_to 'Back', patients_path %>
patients/index.html.erb
patients/show.html.erb
Patients & their Doctors: in the browser
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localhost:3000/patients
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localhost:3000/patients/1
You did it!
You created a many-to-many association!
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Rails Associations
By argroch
Rails Associations
Creating links between your Resource databases.
- 1,433