Rails: eager load on fetching associations, not on find - ruby-on-rails

I've got three nested models: user has many plates and plate has many fruits. I also have a current_user helper method that runs in the before filter to provide authentication. So when I get to my controller, I already have my user object. How can I load all the user's plates and fruits at once?
In other words, I'd like to do something like:
#plates = current_user.plates(include: :fruits)
How can I achieve this?
I'm using Rails 3.1.3.

You will probably want to use the provided #includes method on your relation. DO NOT USE #all unless you intend to immediately work through the records, it will immediately defeat many forms of caching.
Perhaps something like: #plates = current_user.plates.includes(:fruits)
Unfortunately, there are portions of the Rails API that are not as well documented as they should be. I would recommend checking out the following resources if you have any further questions about the Rails query interface:
Query Interface Guide
ActiveRecord::Relation Walkthrough (screencast)
The query interface is possibly the most difficult part of the Rails stack to keep up with, especially with the changes made with Rails 3.0 and 3.1.

You can do
ActiveRecord::Associations::Preloader.new([current_user], :plates => :fruit).run
To eager load associations after current_user was loased. The second argument can be anything you would normally pass to includes: a symbol, an array of symbols, a hash etc

#plates = current_user.plates.all(:include => :fruits)
should do it.

Related

Dry up Rails Active Record query conditions

In my Ruby on Rails project, I have a mailer that basically prepares a daily digest of things that happened in the system for a given user. In the mailer controller, I am gathering all the relevant records from the various models according to some common pattern (within a certain date, not authored by this user, not flagged, etc) and with minor differences from model to model.
There are half a dozen of models involved here (and counting), and most of them have unified column names for certain things (like date of publishing, or whether an item is flagged by admin or not). Hence, the 'where's that go into query are mostly the same. There are minor differences in conditions, but at least 2 or 3 conditions are exactly the same. I easily assume there may be even more similar conditions between models, since we are just starting the feature and haven't figured out the eventual shape of the data yet.
I basically chain the 'where' calls upon each model. It irritates me to have 6 lines of code so close to each other, spanning so far to the right of my code editor, and yet so similar. I am dreaded by the idea that at some point we will have to change one of the 'core' conditions, munging with that many lines of code all at once.
What I'd love to do is to move a core set of conditions that goes into each query into some sort of Proc or whatever, then simply call it upon each model like a scope, and after that continue the 'where' chain with model-specific conditions. Much like a scope on each model.
What I am struggling with is how exactly to do that, while keeping the code inside mailer. I certainly know that I can declare a complex scope inside a concern, then mix it into my models and start each of queries with that scope. However, this way the logic will go away from the mailer into an uncharted territory of model concerns, and also it will complicate each model with a scope that is currently only needed for one little mailer in a huge system. Also, for some queries, a set of details from User model is required for a query, and I don't want each of my models to handle User.
I like the way scopes are defined in the Active Record models via lambdas (like scope :pending, -> { where(approved: [nil, false]) }), and was looking for a way to use similar syntax outside model class and inside my mailer method (possibly with a tap or something like that), but I haven't found any good examples of such an approach.
So, is it possible to achieve? Can I collect the core 'where' calls inside some variable in my mailer method and apply them to many models, while still being able to continue the where chain after that?
The beauty of Arel, the technology behind ActiveRecord query-building, is it's all completely composable, using ordinary ruby.
Do I understand your question right that this is what you want to do?
def add_on_something(arel_scope)
arel_scope.where("magic = true").where("something = 1")
end
add_on_something(User).where("more").order("whatever").limit(10)
add_on_something( Project.where("whatever") ).order("something")
Just ordinary ruby method will do it, you don't need a special AR feature. Because AR scopes are already composable.
You could do something like:
#report_a = default_scope(ModelA)
#report_b = default_scope(ModelB)
private
def default_scope(model)
model.
where(approved: [nil, false]).
order(:created_at)
# ...
end

Get the mobile number of the user in MongoDB [duplicate]

For performance reason, I use as often as possible the only() keyword when writing up a mongoid query in order to specify the fields I want to load.
The usual suspect, is for instance when I want a user's email of all my admins only for display purposes.
I would write:
User.where(:groups => :admins).only(:email).each do |u|
puts u.email
end
I do this because my user model are quite full of a lot of data that I can gladly ignore when listing a bunch of emails.
However, now let imagine, that my users are referenced via a Project model, so that for each project I can do: project.user. Thanks to mongoid's lazy loading, my object user will only get instantiated (and queried from the DB) when I call upon the reference.
But what if I want to list all the email of the owner of all admin project for instance ?
I would write this:
Project.where(:admin_type => true).each do |p|
puts p.user.email
end
The major problem here is that doing this, I load the entire user object for each projects, and if there are lots of project matching the query that could get pretty heavy. So how do I load only the emails ?
I could do this:
User.where(:_id => p.user_id).only(:email).first.email
But this obviously defeat the purpose of the nice syntax of simply doing:
p.user.email
I wish I could write something like: p.user.only(:email).email, but I can't. Any ideas ?
Alex
Answer from creator of Mongoid. It's not possible yet. It's been added as feature request.
I think you need to denormalize here. First of all, read A Note on Denormalization.
You can implement denormalization by self using mongoid events or use great mongoid_denormalize gem. It pretty straight and after implementing it you could use p.user_email or something in your queries.

Caching bunch of simple queries in rails

In my app there're objects, and they belong to countries, regions, cities, types, groups, companies and other sets. Every set is rather simple - it has id, name and sometimes some pointers to other sets, and it never changes. Some sets are small and I load them in before_filter like that:
#countries = Country.all
#regions = Region.all
But then I call, for example,
offer.country.name
or
region.country.name
and my app performs a separate db query-by-id, although I've already loaded them all. After that I perform query through :include, and this case ids, generated by eager loading, do not depend on either I've already loaded this data with another query-by-id or not.
So I want some cache. For example, I may generate hashes with keys as records-ids in my before_filter and then call #countries[offer.country_id].name. This case it seems I don't need eager loading and it's easy turn on Rails.cache here. But maybe there's some smart built-in rails solution that does not require to rewrite everything?
Caching lists of models like that won't cache individual instances of that exist in other model's associations.
The Rails team has worked on implementing Identity Maps in Rails 3.1 to solve this exact problem, but it is disabled by default for now. You can enable it and see if it works for your problem.

Making to_json much more efficient for nested objects

Consider a typical relational structure:
Answer has many comments. comments belongs to a user, and have many likers
answers.to_json(:include => {:comments => {:include => :user, :likers}})
If i call to_json , it will perform the database retrievals one by one for individual user in individual comments.
The much more efficient solution would be to put the required ids for each type in an array, do 3 database calls to retrieve them, put it into a hash, and construct the json according to the hash.
This seems like a pretty common use case. I am thinking of writing my own recursive function for this, but if anyone can point me to something that has already been done, that will be great!
** i am using mongoid / mongodb, i am not sure if it works the same way in active record.
May be you can try mongoid eager loading, then you can query mongo just like AR eager loading
Answer.includes(:comments, :likers)
The patch is still not included in the mongoid master branch, but you can download it as a independednt gem from here
Read this pull request for more info

Rails add custom eager load

I have a number of custom find_by_sql queries in Rails. I would like to use eager loading with them but there doesn't seem to be a good way to do this.
I have seen the eager_custom.rb file floating around and it doesn't seem to work with Rails now. It appear Rails does eager loading differently now, using 2 queries (the regular query plus a query where the 'id IN' the ids from the first query), instead of the single join query used in the past.
My question is if I do a custom SQL query, then do 'id IN' query, is there a way to add back associated objects into the initial query results?
For example I have topics loaded with find_by_sql, then I find topic images where the topic id is in the topics ids, is there a way to add the images manually back to the topics?
Thanks
As you noticed, in Rails 2.1 a new kind of eager/pre-loading was introduced which uses multiple queries with id IN (...). This method is usually faster, especially when there are multiple associations being pre-loaded. You can use this functionality manually with find_by_sql by using the preload_associations class method inherited from ActiveRecord (not recommended). For example:
class Person
def self.find_a_special_group
people = find_by_sql("...")
preload_associations(people, [:jobs, :addresses])
return people
end
end
The preload_associations method is protected, so you must call it from within the class, and it takes (1) an array of objects, (2) an array, hash, or symbol of associations (same format as find's :include option), and (3) an options hash. See the documentation for the ActiveRecord::AssociationPreload::ClassMethods module for more details.
However, having said all of that, this technique is certainly undesirable as the Rails documentation discourages programmers from using preload_associations directly. Are you sure you have to use find_by_sql? Are you sure you know all of the options find takes? (:select, :from, :joins, :group, :having, etc) I'm not saying you don't need find_by_sql, but it might be worth a few minutes to make sure.

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