N:M find confusion

Rails
1

Hello!

I have an N:M mapping of tables. There are three tables involved:

constraints
id
name

people
id
first_name
last_name

constraints_people
constraint_id
person_id

Here is some Ruby code that I have created in order to make the
mapping:

class Person < ActiveRecode::Base
has_and_belongs_to_many: constraints, :class_name => “Constraint”
end

class Constraint < ActiveRecod::Base
has_and_belongs_to_many: people, :class_name => “Person”
end

I would now like to get an array of people who have a constraint id of
e.g. 1. I don’t know how to get this result, I have now tried the
following:

sql_result = Person.find_by_sql("select person_id from
constraints_people where constraint_id = " + constraint.id.to_s)
@people = Person.find(sql_result)

But the sql_result is an assocative array and I only want the IDs. Is
it the correct way to do this with find_by_sql() or is there a better
way to get the @people filled?

Thanks!
Christoph

2

Hi Cristoph.

Having defined the habtm relationship between Constraints and People,
you have several methods to navigate through them. In particular, you
have a people method in the Contraint class, so you can do

contraint.people

to get the people associated to constraint. To go the other way and find
the constraints for a given person

person.constraints

So, to get the people associated to constraint with ID 1 you just need
to find the constraint by the id and ask it for its people

Constraint.find(1).people

In general, you wouldn’t use find_by_sql but in special cases.

By the way, since you have named both tables (plural) and classes
(singular) according to Ror standars you don’t need to specify the
:class_name param in the habtm declarations, you have the default.

Regards

[email protected] escribi�:

first_name
has_and_belongs_to_many: constraints, :class_name => “Constraint”
sql_result = Person.find_by_sql("select person_id from

–
Andr�s Cirugeda E.
ASPgems
Email: andres at aspgems dot com

‘All we have to decide is what to do with the time that is given to us’.
Gandalf.

3

Ouch… hurts to see the beginners use find_by_sql and such. Poor
php/java developers aren’t used to the beauty of ruby =(

Read the beginners guides here: http://wiki.rubyonrails.com/rails

4

[email protected] wrote:

first_name
has_and_belongs_to_many: constraints, :class_name => “Constraint”
sql_result = Person.find_by_sql("select person_id from
constraints_people where constraint_id = " + constraint.id.to_s)
@people = Person.find(sql_result)

But the sql_result is an assocative array and I only want the IDs. Is
it the correct way to do this with find_by_sql() or is there a better
way to get the @people filled?

All you need is

@people = constraint.people

given that you’ve found constraint using something like

constraint = Constraint.find(1)

–
We develop, watch us RoR, in numbers too big to ignore.