Question

I have two tables: interests and Link_ui (this is for recording user and interest)
I want to input the user id and show all interests name that user have.

In Link_ui controller:

 def output
    @interests = LinkUi.find_by_sql [ 'SELECT interests.name FROM link_uis, interests
    WHERE link_uis.interest_id = interests.id AND link_uis.user_id=? ', params['user_id'] ]

And input page:

<%= form_tag :action => 'output', :method => 'post' %>
  enter id.
  <%= text_field_tag ':user_id', '', 'size' => 30 %>

It comes out nothing, but I am sure there is matched data in database. And if I don’t input parameter just set link_uis.user_id = 1, it comes out:

your search are [#<LinkUi >, #<LinkUi >, #<LinkUi >, #<LinkUi >]

What’s wrong with this..

Answer 1

Well, find_by_sql on a LinkUi model expects you to return columns from the link_uis table, whereas you’re selecting just interests.name. However, you are picking a bit of a fight with ActiveRecord, there. 🙂

You usually want to avoid find_by_sql, and instead let ActiveRecord generate your SQL for you. Probably most important for your example are associations.

The way I see it, you have a bunch of Users, and a bunch of Interests. Your LinkUis tie these two together (a LinkUi belongs to a User and an Interest). Feel free to correct me on this; this is your business logic as I gather from your example.

These classes (whose names I’ve emphasized) are your models, defined in the app/models directory. The assocations (relationships) between them should be defined on those classes.

---

Start of with a simple association in your User model:

class User < ActiveRecord::Base
  has_many :link_uis
end

And in your Interest model:

class Interest < ActiveRecord::Base
  has_many :link_uis
end

Then the LinkUi model that ties it together:

class LinkUi < ActiveRecord::Base
  belongs_to :user
  belongs_to :interest
end

Now, given any User, you can get his/her LinkUis by simply saying user.link_uis.all, and for each LinkUi, you can get the Interest as link_ui.interest. You can tell ActiveRecord to try and fetch these two in one shot as efficiently as possible using :include, and get a list of Interest names using the standard Ruby collect method. It then becomes:

user = User.find params['user_id']
link_uis = user.link_uis.all(:include => :interest)
interest_names = link_uis.collect { |link_ui| link_ui.interest.name }

---

You can take it one step further; for any User, you can directly get his/her Interests. Once you’ve set up the above associations, you can fold two ‘steps’ into one, like this:

class User < ActiveRecord::Base
  has_many :link_uis
  has_many :interests, :through => :link_uis
end

Which could turn the example into this one-liner:

interest_names = User.find(params[:user_id]).interests.collect { |i| i.name }