Let's work with these classes:
class User < ActiveRecord::Base
has_many :project_participations
has_many :projects, through: :project_participations, inverse_of: :users
end
class ProjectParticipation < ActiveRecord::Base
belongs_to :user
belongs_to :project
enum role: { member: 0, manager: 1 }
end
class Project < ActiveRecord::Base
has_many :project_participations
has_many :users, through: :project_participations, inverse_of: :projects
end
A user can participate in many projects with a role as a member or a manager. The connecting model is called ProjectParticipation.
I now have a problem using the associations on unsaved objects. The following commands work like I think they should work:
# first example
u = User.new
p = Project.new
u.projects << p
u.projects
=> #<ActiveRecord::Associations::CollectionProxy [#<Project id: nil>]>
u.project_participations
=> #<ActiveRecord::Associations::CollectionProxy [#<ProjectParticipation id: nil, user_id: nil, project_id: nil, role: nil>]>
So far so good - AR created the ProjectParticipation by itself and I can access the projects of a user with u.projects.
But it does not work if I create the ProjectParticipation by myself:
# second example
u = User.new
pp = ProjectParticipation.new
p = Project.new
pp.project = p # assign project to project_participation
u.project_participations << pp # assign project_participation to user
u.project_participations
=> #<ActiveRecord::Associations::CollectionProxy [#<ProjectParticipation id: nil, user_id: nil, project_id: nil, role: nil>]>
u.projects
=> #<ActiveRecord::Associations::CollectionProxy []>
Why are the projects empty? I cannot access the projects by u.projects like before.
But if I go through the participations directly, the project shows up:
u.project_participations.map(&:project)
=> [#<Project id: nil>]
Shouldn't it work like the first example directly: u.projects returning me all projects not depending on whether I create the join object by myself or not? Or how can I make AR aware of this?
Short answer: No, second example won't work the way it worked in first example. You must use first example's way of creating intermediate associations directly with user and project objects.
Long answer:
Before we start, we should know how has_many :through is being handled in ActiveRecord::Base. So, let's start with has_many(name, scope = nil, options = {}, &extension) method which calls its association builder here, at the end of method the returned reflection and then add reflection to a hash as a cache with key-value pair here.
Now question is, how do these associations gets activated?!?!
It's because of association(name) method. Which calls association_class method, which actually calls and return this constant: Associations::HasManyThroughAssociation, that makes this line to autoload active_record/associations/has_many_through_association.rb and instantiate its instance here. This is where owner and reflection are saved when the association is being created and in the next reset method is being called which gets invoked in the subclass ActiveRecord::Associations::CollectionAssociation here.
Why this reset call was important? Because, it sets #target as an array. This #target is the array where all associated objects are stored when you make a query and then used as cache when you reuse it in your code instead of making a new query. That's why calling user.projects(where user and projects persists in db, i.e. calling: user = User.find(1) and then user.projects) will make a db query and calling it again won't.
So, when you make a reader call on an association, e.g.: user.projects, it invokes the collectionProxy, before populating the #target from load_target.
This is barely scratching the surface. But, you get the idea how associations are being build using builders(which creates different reflection based on the condition) and creates proxies for reading data in the target variable.
tl;dr
The difference between your first and second examples is the way their association builders are being invoked for creating associations' reflection(based on macro), proxy and target instance variables.
First example:
u = User.new
p = Project.new
u.projects << p
u.association(:projects)
#=> ActiveRecord::Associations::HasManyThroughAssociation object
#=> #proxy = #<ActiveRecord::Associations::CollectionProxy [#<Project id: nil, name: nil, created_at: nil, updated_at: nil>]>
#=> #target = [#<Project id: nil, name: nil, created_at: nil, updated_at: nil>]
u.association(:project_participations)
#=> ActiveRecord::Associations::HasManyAssociation object
#=> #proxy = #<ActiveRecord::Associations::CollectionProxy [#<ProjectParticipation id: nil, user_id: nil, project_id: nil, role: nil, created_at: nil, updated_at: nil>]>
#=> #target = [#<ProjectParticipation id: nil, user_id: nil, project_id: nil, role: nil, created_at: nil, updated_at: nil>]
u.project_participations.first.association(:project)
#=> ActiveRecord::Associations::BelongsToAssociation object
#=> #target = #<Project id: nil, name: nil, created_at: nil, updated_at: nil>
Second example:
u = User.new
pp = ProjectParticipation.new
p = Project.new
pp.project = p # assign project to project_participation
u.project_participations << pp # assign project_participation to user
u.association(:projects)
#=> ActiveRecord::Associations::HasManyThroughAssociation object
#=> #proxy = nil
#=> #target = []
u.association(:project_participations)
#=> ActiveRecord::Associations::HasManyAssociation object
#=> #proxy = #<ActiveRecord::Associations::CollectionProxy [#<ProjectParticipation id: nil, user_id: nil, project_id: nil, role: nil, created_at: nil, updated_at: nil>
#=> #target = [#<ProjectParticipation id: nil, user_id: nil, project_id: nil, role: nil, created_at: nil, updated_at: nil>]
u.project_participations.first.association(:project)
#=> ActiveRecord::Associations::BelongsToAssociation object
#=> #target = #<Project id: nil, name: nil, created_at: nil, updated_at: nil>
There's no proxy for BelongsToAssociation, it has just target and owner.
However, if you are really inclined to make your second example work, you will just have to do this:
u.association(:projects).instance_variable_set('#target', [p])
And now:
u.projects
#=> #<ActiveRecord::Associations::CollectionProxy [#<Project id: nil, name: nil, created_at: nil, updated_at: nil>]>
In my opinion which is a very bad way of creating/saving associations. So, stick with the first example itself.
This is more of a rails structure thing at the level of the ruby data structures.
To simplify it lets put it this way.
First of all imagine User as a data structure contains:
project_participations Array
projects Array
And Project
users Array
project_participations Array
Now when you mark a relation to be :through another (user.projects through user.project_participations)
Rails implies that when you add a record to that first relation (user.projects) it will have to create another one in the second realation (user.project_participations) that is all the effect of the 'through' hook
So in this case,
user.projects << project
#will proc the 'through'
#user.project_participations << new_entry
Keep in mind that the project.users is still not updated because its a completely different data structure and you have no reference to it.
So lets take a look what will happen with the second example
u.project_participations << pp
#this has nothing hooked to it so it operates like a normal array
So In conclusion, this acts like a one way binding on a ruby data structure level and whenever you save and refresh your objects, this will behave the way you wanted.
At the risk of some serious oversimplification let me try to explain what is going on
What Most of the other answers are trying to tell you is that these objects have not been linked yet by active record until they are persisted in the DB. Consequently the association behavior that you are expecting is not fully wired up.
Notice that this line from your first example
u.project_participations
=> #<ActiveRecord::Associations::CollectionProxy [#<ProjectParticipation id: nil, user_id: nil, project_id: nil, role: nil>]>
Is identical to the result from your second example
u.project_participations
=> #<ActiveRecord::Associations::CollectionProxy [#<ProjectParticipation id: nil, user_id: nil, project_id: nil, role: nil>]>
This statement from your analysis of what you think rails is doing is inaccurate:
So far so good - AR created the ProjectParticipation by itself and I
can access the projects of a user with u.projects.
AR record has not created the ProjectParticipation. You have declared this relationship in your model. AR is just returning proxy for the collection that it will have at some point in the future, which when populated assigned, etc, you will be be able to iterate over and query its members etc.
The reason that this works:
u.projects << p
u.projects
=> #<ActiveRecord::Associations::CollectionProxy [#<Project id: nil>]>
But this doesn't
pp.project = p # assign project to project_participation
u.project_participations << pp # assign project_participation to user
u.project_participations
=> #<ActiveRecord::Associations::CollectionProxy [#<ProjectParticipation id: nil, user_id: nil, project_id: nil, role: nil>]>
u.projects
=> #<ActiveRecord::Associations::CollectionProxy []>
Is that in the first case you are just adding objects to an array that your user instance has direct access to. In the second example the has_many_through relationship reflects a relationship that happens at the database level. In the second example in order for the your projects to be accessible through your user, AR has to actually run a query that joins the tables and returns the data you are looking for. Since none of these objects is persisted yet that database query can't happen yet so all you get back are the proxies.
The last bit of code is misleading because it is not actually doing what you think.
u.project_participations.map(&:project)
=> [#<Project id: nil>]
In this case you have a user which is directly holding an array of ProjectParticipations one of which is directly holding a project so it works. It is not actually using the has_many_through mechanism in they way you think.
Again this is a bit of an oversimplification but that is the general idea.
Associations are defined on database level and make use of database table's primary key (and in polymorphic cases, class name). In case of has_many :through the lookup on association (say, User's Projects) is:
Fetch all User-Project pairs, whose user_id is a certain value (primary key of an existing User in the database)
Fetch all project_id (primary keys of projects) from these pairs
Fetch all Projects by resulting keys
Of course, these are simple terms, in database terms it's much shorter and uses more complicated abstractions, such as an inner join, but the essence is the same.
When you create a new object via new, it is not yet saved in the database, and therefore has no primary key (it's nil). That said, if the object is not in a database yet, you have no way of referencing it from any ActiveRecord's association.
Side note:
There is a possibility, however, that a newly created (and not saved yet) object will act as if something is associated with it: it might show entries belonging to NULL. This usually means you have an error in your database schema that allows such things to happen, but hypothetically, one could design his database to make use of this.
Related
I have an Invoice and Order model with the following relationship:
class Invoice < ApplicationRecord
has_many orders
end
class Order < ApplicationRecord
belongs_to invoice
end
In my Invoice model, I am trying to loop through the orders belonging to an invoice using collect. An extra order with nil attributes is being added to my results.
orders.collect{|order| order}
Results:
[#<Order id: 1, menu_item_id: 1, invoice_id: 1, quantity: 1, status: "Submitted", created_at: "2020-05-25 15:48:25", updated_at: "2020-05-25 15:48:25">,
#<Order id: nil, menu_item_id: nil, invoice_id: 1, quantity: nil, status: nil, created_at: nil, updated_at: nil>]
I have checked the existence of this blank item in the database but I am not seeing anything.
Edit:
Fixed this by removing an unnecessary instance.order.build in my controller. Thanks to #3limin4t0r for bringing this up
Things don't magically appear. If there is an empty record in your orders collection, you've probably added it somewhere to the collection.
Things like:
invoice.orders.build
invoice.orders.new
invoice.orders << Order.new
All add an empty order to the orders collection. These actions are commonly found inside the controller new and create actions, but can also appear in other places.
Once a collection like invoice.orders is loaded it will stay loaded for the duration of the request (as long as invoices is in scope and you're working on the same invoice instance).
The best thing to do is look where you've added this additional instance to the collection and remove the statement if possible. Alternatively you can reload the collection from the database with invoice.orders.reload resetting the orders collection.
When duplicating an object with object_dup = object.dup, all associations are copied.
object_dup.foos == object.foos
I would like to duplicate/clone object without its associations, or delete the associations all after duplication. I would like to destroy all duplicated associations on object_dup. It might be easier to just create a new object, but duplication saves me from property-setting-hell.
Is that possible?
Actually .dup method didn't duplicate associations, it just copy the foreign key (parents).
Examples:
# Original
my_post = Post.first
=> #<Post id: 1, title: 'blabla', category_id: 10>
# Duplicate
my_post.dup
=> #<Post id: nil, title: 'blabla', category_id: 10>
# Have the same category_id (10)
My best way to duplicate without some attributes :
Post.new(my_post.attributes.slice('titles'))
=> #<Post id: nil, title: 'blabla', category_id: nil>
Here we creating a new empty Post, get original post attributes with my_post.attributes and slice only attributes we want with slice('title') (accept multiples attributes, examples: slice('title', 'content', 'tags'))
.dup Documentation
.slice Documentation
I have a model with a has_many relationship that's built through nested forms:
class User < ActiveRecord::Base
has_many :properties
accepts_nested_attributes_for :properties, allow_destroy: true
def billing_address
debugger
properties.find_by(billing_address: true)
end
end
Overall, the relationship and nested form works. However, if I call the method billing_address during creation, then it returns nil, even if there is a nested property with billing_address set to true. I experimented with this in debugger, and it seems that calling properties.find_by or properties.where during creation always results in nil, even if the parameters match a real object.
When I type properties into debugger, I get results like this, which clearly show that there is a property with billing_address set to true:
#<ActiveRecord::Associations::CollectionProxy [#<User::Property id: nil, address: "1111 E 1st", city: "Austin", state: "TX", zip_code: "11111", phone_number: "11111111111", user_id: nil, primary: true, billing_address: true, created_at: nil, updated_at: nil>]>
So why can't I find it with a query such as properties.find_by(billing_address: true)? Is there another approach to getting at this data?
find_by and where search the database (via SQL queries) and at the time you are checking, the records have not been saved, so they cannot be found there. I know of no way to "look up" the proxies, but if you provide some more information about the context in which you're trying to access them, I might have some ideas.
I'm trying to do something fairly simple. I have two models, User and Group. For simplicity's sake, let's say they look like this:
class User < ActiveRecord::Base
has_and_belongs_to_many :groups
end
and
class Group < ActiveRecord::Base
has_and_belongs_to_many :users
end
Now, for some reason, I have a user that has the same group twice. In the Rails Console:
user = User.find(1000)
=> #<User id: 1000, first_name: "John", last_name: "Doe", active: true, created_at:
"2013-01-02 16:52:36", updated_at: "2013-06-17 16:21:09">
groups = user.groups
=> [#<Group id: 1, name: "student", is_active: true, created_at: "2012-12-24 15:08:59",
updated_at: "2012-12-24 15:08:59">, #<Group id: 1, name: "student", is_active: true,
created_at: "2012-12-24 15:08:59", updated_at: "2012-12-24 15:08:59">]
user.groups = groups.uniq
=> [#<Group id: 1, name: "student", is_active: true, created_at: "2012-12-24 15:08:59",
updated_at: "2012-12-24 15:08:59">]
user.save
=> true
And there is some SQL output that I've silenced. I would think that everything should be all set, but it's not. The groups aren't updated, and that user still has both. I could go into the join table and manually remove the duplicates, but that seems cludgy and gross and unnecessary. What am I doing wrong here?
I'm running Rails 3.2.11 and Ruby 1.9.3p392
Additional note: I've tried this many different ways, including using user.update_attributes, and using group_ids instead of the groups themselves, to no avail.
The reason this doesn't work is because ActiveRecord isn't handling the invalid state of duplicates in the habtm association (or any CollectionAssociation for that matter). Any ids not included in the newly assigned array are deleted - but there aren't any in this case. The relevant code:
# From lib/active_record/associations/collection_association.rb
def replace_records(new_target, original_target)
delete(target - new_target)
unless concat(new_target - target)
#target = original_target
raise RecordNotSaved, "Failed to replace #{reflection.name} because one or more of the " \
"new records could not be saved."
end
target
end
The 'targets' being passed around are Arrays of assigned records. Note the call to delete(target - new_target) is equivalent in your case to delete(user.groups - user.groups.uniq) which results in an empty Array passed (since comparison is based on the id attribute of each record).
Instead, you'll need to clear out the association and then reassign the single group again:
group = user.groups.first
user.groups.clear
user.groups << group
This might be a way to cleanup those duplicates (it handles any number of groups of duplicate associations):
user = User.find(1000)
user.groups << user.groups.group_by(&:id).values.find_all {|v| v.size > 1}.each {|duplicates| duplicates.uniq_by! {|obj| obj.id}}.flatten.each {|duplicate| user.groups.delete(duplicate)}
Postgresql / Rails 3 / Ruby 1.9.2 / OSX
class Deliverable < ActiveRecord::Base
has_many :models, :dependent => :destroy
def build_template
models << sect_letterhead << sect_ica << sect_project_description_exhibit
end
end
ruby-1.9.2-p180 :002 > d.models.first
=> #<Model id: 1, company_id: nil, deliverable_id: 1, model_id: nil, type: nil, kind: "cover_page", csv_file_name: nil, csv_content_type: nil, csv_file_size: nil, csv_updated_at: nil>
ruby-1.9.2-p180 :003 > d.models.first.update_attribute(:csv_file_name, "blah")
=> true
ruby-1.9.2-p180 :004 > d.models.first
=> #<Model id: 5, company_id: nil, deliverable_id: 1, model_id: nil, type: nil, kind: "opening_letter", csv_file_name: nil, csv_content_type: nil, csv_file_size: nil, csv_updated_at: nil>
Whenever I update_attribute a model, that model is pushed to the end of the array. Is there any way other than assigning a custom sort attribute to avoid changing the array order when update_attribute is called? I could fairly easily do this, but I am interested in the behavior of the database.
Even when I remove the timestamps from the model object, the behavior remains.
How does this occur if there are no timestamps to keep track of which model was just updated? Or is the model actually pulled out of the array and put back in the last position when I call update_attribute?
I'm no PostgreSQL expert, but I've noticed the same behaviour. My understanding is that PostgreSQL doesn't have a default order, per se (whereas MySQL is reliable - as far as I know - in ordering by the primary key). PostgreSQL's data format maybe means that modified records are shifted to the end.
In short: this isn't a Rails thing - it's a PostgreSQL thing, hence why timestamps don't matter.
Either way, it forces you to be clear about the order you want items returned in. Not a terrible idea.