Help with many-to-many using :through


#1

I am using many-to-many using :through since I need to store additional
information in the join table.

I am doing this and it seems to work. I want to verify that this is the
best
way to do this.

Tables
foods - id, food
foodallergies - food_id, symptom_id, a few other columns
symptoms - id, symptom

Models
class Food < ActiveRecord::Base
has_many :foodallergies
has_many :symptoms, :through => :foodallergies
end

class Symptom < ActiveRecord::Base
has_many :foodallergies
has_many :foods, :through => :foodallergies
end

class Foodallergy < ActiveRecord::Base
belongs_to :foods
belongs_to :symptoms
end

I have a JournalEntry model that stores foods and symptoms as
comma-seperated strings. Upon record creation in the JournalEntry, I
want to
parse the foods and symptoms and create a many to many relationship
between
them. I do this like this in the create method in the JournalController

def create
params[:journal_entry][“user_id”] = @session[:user].id
@journal_entry = JournalEntry.new(params[:journal_entry])

if @journal_entry.save
  # parse the foods and create a new record if food not already in 

db
params[:journal_entry][“eat”].split(%r{,\s*}).each do |food|
f = Food.find(:first, :conditions => [“food=?”, food.strip])
unless f
f = Food.create(:food => food)
end

    params[:journal_entry]["symptoms"].split(%r{,\s*}).each do 

|symptom|
s = Symptom.find(:first, :conditions => [“symptom=?”,
symptom.strip])
unless s
s = Symptom.create(:symptom => symptom)
end
# create join
f.foodallergies.create(:food_id => f.id, :symptom_id => s.id)
end
end

  flash[:notice] = 'JournalEntry was successfully created.'
  redirect_to :action => 'list'
else
  render :action => 'new'
end

end

Is this the best way to do it? Since I am using create, there is a
database
interaction on every ‘create’. Is there a way to leverage ActiveRecord
functionality to batch the database interaction?

Thanks in advance,
Vaishal


#2

I can’t even get :through to work.

I get this error: ‘Unknown key(s): through’

Can anyone help?

Thanks!


#3

Thanks Josh. Nice example on your blog. May I suggest a tutorial on
data
entry using through with examples? A lot of people have posted examples
about how to retrieve data using :through but I couldn’t find any on
data
entry issues.

Transactions…aha, I will try that out.

BTW, if instead of using create, I used the new method for foods,
symptoms
and foodallergies, would that work? Would rails then save everything in
a
batch? or would it not even allow me to do it because just using new
will
not get me an id for food or symptom and hence it wouldn’t know what to
insert in the association?

Vaishal


#4

Vaishal S. wrote:

I am using many-to-many using :through since I need to store additional
information in the join table.

I am doing this and it seems to work. I want to verify that this is the
best
way to do this.

Your has_many :through code looks solid to me. You might want to check
the association examples on my blog if you run into trouble. I’ll be
doing another one soon for working with attributes in a join model.

Is this the best way to do it? Since I am using create, there is a database
interaction on every ‘create’. Is there a way to leverage ActiveRecord
functionality to batch the database interaction?

Wrap all the calls in a transaction. (JournalEntry.transaction do …
end) When you commit the transaction all the new records will be saved
en masse. You may also be able to get some leverage by using build()
instead of create(), then when you call save on the parent object the
children objects should be saved along with. I haven’t used the latter
feature with :through associations yet, so I’m not sure how well that
works. But the transaction thing should help a lot.

By the way, I suggest keeping the user_id in the session, not the user
object itself. It will save you a world of trouble trying to keep things
in sync.


Josh S.
http://blog.hasmanythrough.com


#5

are you are using Rails 1.1?