Can someone clarify this for me? Suppose I have two tables like:
class Company < ActiveRecord::Base
belongs_to :category
end
class Category < ActiveRecord::Base
has_many :companies
end
I understand that the belongs_to causes related companies to be read
from the db when I ask for a category. I suspect SQL something like
select * from categories, companies
where category.id = myid
and companies.category_id = category.id
but what SQL results from the has_many?
--
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You can look at the development.log file to see all the SQL queries made
but it''s probably like the query you said:
belongs_to:
select * from categories where category_id = {company_category_id}
- where category id is retrived from the company results
has_many:
select * from companies where category_id = {category_id}
- where category id is the id of the retrived category
Steve
Richard Williams wrote:> Can someone clarify this for me? Suppose I have two tables like:
>
> class Company < ActiveRecord::Base
> belongs_to :category
> end
>
> class Category < ActiveRecord::Base
> has_many :companies
> end
>
> I understand that the belongs_to causes related companies to be read
> from the db when I ask for a category. I suspect SQL something like
>
> select * from categories, companies
> where category.id = myid
> and companies.category_id = category.id
>
> but what SQL results from the has_many?
>
On May 11, 2006, at 10:08 AM, Richard Williams wrote:> Can someone clarify this for me? Suppose I have two tables like: > > class Company < ActiveRecord::Base > belongs_to :category > end > > class Category < ActiveRecord::Base > has_many :companies > end > > I understand that the belongs_to causes related companies to be read > from the db when I ask for a category. I suspect SQL something like > > select * from categories, companies > where category.id = myid > and companies.category_id = category.id > > but what SQL results from the has_many?I would say that you are thinking about this in the wrong way. Technically the belong_to and the has_many methods don''t specifically create SQL. What they do is create a programatic link between the two models that is then expressed as SQL when you request specific data from the database. If you do: Company.find(:first), say through script/console, SQL is generated, but no SQL specific to the belongs_to relationship. If you do: Company.find(:first).category, you actually create two SQL requests to the database. The first is to return the first Company record. The second is to return the parent Category record of that Company record. Likewise, if you do: Category.find(:first).companies, you get 2 SQL requests, the second of which brings back an array of child Company records. If you want a deeper understanding of this, there are several things you should do. First, I recommend getting the Agile Web Development with Rails book (http://pragmaticprogrammer.com/). Second, you should read through the API docs on this page: <http://rails.rubyonrails.com/classes/ActiveRecord/Associations/ ClassMethods.html> Third, you should launch your Rails app under Development, with the ability to see the WEBrick/lighttpd output, launch script/console and give some find calls a try, looking at the web server output. In that output will be the SQL generated by Rails. -Brian
On Thursday, May 11, 2006, at 3:15 PM, Stephen Bartholomew wrote:>You can look at the development.log file to see all the SQL queries made >but it''s probably like the query you said: > >belongs_to: >select * from categories where category_id = {company_category_id} >- where category id is retrived from the company results > >has_many: >select * from companies where category_id = {category_id} >- where category id is the id of the retrived category > >Steve > > >Richard Williams wrote: >> Can someone clarify this for me? Suppose I have two tables like: >> >> class Company < ActiveRecord::Base >> belongs_to :category >> end >> >> class Category < ActiveRecord::Base >> has_many :companies >> end >> >> I understand that the belongs_to causes related companies to be read >> from the db when I ask for a category. I suspect SQL something like >> >> select * from categories, companies >> where category.id = myid >> and companies.category_id = category.id >> >> but what SQL results from the has_many? >> >_______________________________________________ >Rails mailing list >Rails@lists.rubyonrails.org >http://lists.rubyonrails.org/mailman/listinfo/railsSo I guess that if you don''t ever want to do the has_many query, i.e. you don''t every want to get a list of categories for a particular set of companies you would not need to have the has_many association in the model. -- Posted with http://DevLists.com. Sign up and save your mailbox.
On May 11, 2006, at 10:29 AM, Richard Williams wrote:> On Thursday, May 11, 2006, at 3:15 PM, Stephen Bartholomew wrote: >> You can look at the development.log file to see all the SQL >> queries made >> but it''s probably like the query you said: >> >> belongs_to: >> select * from categories where category_id = {company_category_id} >> - where category id is retrived from the company results >> >> has_many: >> select * from companies where category_id = {category_id} >> - where category id is the id of the retrived category > > So I guess that if you don''t ever want to do the has_many query, i.e. > you don''t every want to get a list of categories for a particular > set of > companies you would not need to have the has_many association in > the model.Not true, IMO. Unfortunately, while Stephen''s description is essentially accurate, it''s too simplistic a view into what ActiveRecord associations can do for you. If you have a relationship where it makes sense to say one model belongs_to another model, the "parent" model should have the appropriate has_* association specified within it. I believe it would technically work, if you only specified one side of the association, you would be cutting off half (or more) of the functionality that ActiveRecord is trying to provide. I would say it''s actually irrelevant if the code you are currently planning to develop only accesses the association from one side, you want to have both sides specified so that when you need both sides, it will be there. -Brian
As Brian said, has_many defines a relationship - it doesn''t generate SQL. There is more involved in this relationship that just selecting out children, such as creating and deleting related records. Also, the query to select children wont be run unless you try to do something with them: for company in @category.companies # ... code end I also second all of Brian''s advice. Cheers, Steve Richard Williams wrote:> On Thursday, May 11, 2006, at 3:15 PM, Stephen Bartholomew wrote: > >>You can look at the development.log file to see all the SQL queries made >>but it''s probably like the query you said: >> >>belongs_to: >>select * from categories where category_id = {company_category_id} >>- where category id is retrived from the company results >> >>has_many: >>select * from companies where category_id = {category_id} >>- where category id is the id of the retrived category >> >>Steve >> >> >>Richard Williams wrote: >> >>>Can someone clarify this for me? Suppose I have two tables like: >>> >>>class Company < ActiveRecord::Base >>> belongs_to :category >>>end >>> >>>class Category < ActiveRecord::Base >>> has_many :companies >>>end >>> >>>I understand that the belongs_to causes related companies to be read >>>from the db when I ask for a category. I suspect SQL something like >>> >>> select * from categories, companies >>> where category.id = myid >>> and companies.category_id = category.id >>> >>>but what SQL results from the has_many? >>> >> >>_______________________________________________ >>Rails mailing list >>Rails@lists.rubyonrails.org >>http://lists.rubyonrails.org/mailman/listinfo/rails > > > > So I guess that if you don''t ever want to do the has_many query, i.e. > you don''t every want to get a list of categories for a particular set of > companies you would not need to have the has_many association in the model. > >
Thanks Brian. What I was confused about was that I _never_ see example model where ONLY a has_many or ONLY a belongs to is shown. Always they are shown as a pair. I wanted to know if it is "normal" to use only one or the other and when that would be. I think I understand now that you can use either one independently of the other or you can use both together as required by the application. -- Posted with http://DevLists.com. Sign up and save your mailbox.