How can i remove the last element from an ActiveRecord_Relation in rails?
e.g. if I set:
#drivers = Driver.all
I can add a another Driver object called #new_driver to #drivers by doing:
#drivers << #new_driver
But how can I remove an object from #drivers?
The delete method doesn't seem to work, i.e.
#drivers.delete(0)
You can use the reject! method, this will remove the object from the collection without affecting the db
for example:
driver_to_delete = #driver.first # you need the object that you want removed
#drivers.reject!{|driver| driver == driver_to_delete}
Very late too, but I arrived here looking for a fast answer and finished by thinking by myself ;)
Just to clarify about the different answers and the Rails 6.1 comment on accepted answer:
The OP wanted to remove one entry from a query, but NOT remove it from database, so any answer with delete or destroy is just wrong (this WILL delete data from your database !!).
In Ruby (and therefore Rails) convention, shebang methods (ending with !) tend to alter the given parameter. So reject! would imply modifying the source list ... but an ActiveRecord_Relation is basically just a query, NOT an array of entries !
So you'd have 2 options:
Write your query differently to specifically say you don't want some id:
#drivers.where.not(id: #driver_to_remove) # This still is an ActiveRecord_Relation
Use reject (NO shebang) on your query to transform it into an Array and "manually" remove the entry you don't want:
#drivers.reject{ |driver| driver == #driver_to_remove}
# The `reject` forces the execution of the query in DB and returns an Array)
On a performance point of view, I would personally recommend the first solution as it would be just a little more complex against the DB where the latter implies looping on the whole (eventually large) array.
Late to the question, but just had the same issue and hope this helps someone else.
reject!did not work for ActiveRecord_Relation in Rails 4.2
drop(1) was the solution
In this case #drivers.drop(0) would work to drop the first element of the relation
Since its an array of objects, have you tried to write something like #drivers.delete(#new_driver) or #drivers.delete(id: #new_driver.id) ?
This is the documentation you need:
#group.avatars << Avatar.new
#group.avatars.delete(#group.avatars.last)
--
.destroy
The problem you've got is you're trying to use collection methods on a non-collection object. You'll need to use the .destroy ActiveRecord method to get rid of the record from the database (and consequently the collection):
#drivers = Driver.all
#drivers.last.destroy
--
Scope
.delete will remove the record from the DB
If you want to pull specific elements from the db to populate the #drivers object, you'll need to use a scope:
#app/models/driver.rb
Class Driver < ActiveRecord::Base
scope :your_scope, -> { where column: "value" }
end
This will allow you to call:
#app/controllers/drivers_controller.rb
def index
#drivers = Driver.your_scope
end
I think you're getting the MVC programming pattern confused - data manipulation is meant to happen in the model, not the controller
As stated above, reject! doesn't work in Rails 4.2, but delete does, so #drivers.delete(#new_driver) works, and more generally:
#drivers.delete(Driver.where(your condition))
Related
I'm not sure if this is just a lacking of the Rails language, or if I am searching all the wrong things here on Stack Overflow, but I cannot find out how to add an attribute to each record in an array.
Here is an example of what I'm trying to do:
#news_stories.each do |individual_news_story|
#user_for_record = User.where(:id => individual_news_story[:user_id]).pluck('name', 'profile_image_url');
individual_news_story.attributes(:author_name) = #user_for_record[0][0]
individual_news_story.attributes(:author_avatar) = #user_for_record[0][1]
end
Any ideas?
If the NewsStory model (or whatever its name is) has a belongs_to relationship to User, then you don't have to do any of this. You can access the attributes of the associated User directly:
#news_stories.each do |news_story|
news_story.user.name # gives you the name of the associated user
news_story.user.profile_image_url # same for the avatar
end
To avoid an N+1 query, you can preload the associated user record for every news story at once by using includes in the NewsStory query:
NewsStory.includes(:user)... # rest of the query
If you do this, you won't need the #user_for_record query — Rails will do the heavy lifting for you, and you could even see a performance improvement, thanks to not issuing a separate pluck query for every single news story in the collection.
If you need to have those extra attributes there regardless:
You can select them as extra attributes in your NewsStory query:
NewsStory.
includes(:user).
joins(:user).
select([
NewsStory.arel_table[Arel.star],
User.arel_table[:name].as("author_name"),
User.arel_table[:profile_image_url].as("author_avatar"),
]).
where(...) # rest of the query
It looks like you're trying to cache the name and avatar of the user on the NewsStory model, in which case, what you want is this:
#news_stories.each do |individual_news_story|
user_for_record = User.find(individual_news_story.user_id)
individual_news_story.author_name = user_for_record.name
individual_news_story.author_avatar = user_for_record.profile_image_url
end
A couple of notes.
I've used find instead of where. find returns a single record identified by it's primary key (id); where returns an array of records. There are definitely more efficient ways to do this -- eager-loading, for one -- but since you're just starting out, I think it's more important to learn the basics before you dig into the advanced stuff to make things more performant.
I've gotten rid of the pluck call, because here again, you're just learning and pluck is a performance optimization useful when you're working with large amounts of data, and if that's what you're doing then activerecord has a batch api you should look into.
I've changed #user_for_record to user_for_record. The # denote instance variables in ruby. Instance variables are shared and accessible from any instance method in an instance of a class. In this case, all you need is a local variable.
In my rails app I have a collectionproxy that is an array (I think) of records. I want to take a record from the middle of the array and put it at the beginning. I don't know the position of the item, but I can find it using an attribute on user model. It seems like some methods aren't available to use on a collectionproxy.
I tried:
user_images = user.images
user_images.insert(0, user_images.delete(user.images.find_by_id(user.primary_image_id))
but got an error that I gave 2 arguments but it expected 1. I'm guessing because the insert method that is used on arrays isnt the same method that is used on collectionproxies. What's the best way to do this?
Edit: I just need this to display the items in the view, I don't need to change at the database level.
As very few methods are available for collection proxy, first change the collection proxy to array and then manipulate it.
Here is the code to do so,
user_images = user.images.to_a //converted collection to array
user_images.unshift(user_images.detect{ |image| image.id == user.primary_image_id}).uniq //used unshift
puts user_images
The magic we have done here is detect the images that's the primary image of the user and unshift into array
The unshift adds the object in the beginning.
Now remove the duplicated oject which is already there at someplace by using uniq.
That's it your required objects comes first into the array and you can use this in the view as active record collection is used.
It looks like there's currently no way to add to the beginning of a CollectionProxy. The prepend and sort methods were removed. Here's the API for the deprecated prepend method:
http://apidock.com/rails/v4.2.1/ActiveRecord/Associations/CollectionProxy/prepend
You could re-think this slightly, and use the append or << operator along with delete to copy elements to the end of the collection, and delete them from the middle. It's not ideal, but it might be a workaround until you have a better solution.
Hi I currently have a helper method that gets Klass.all.map{|m| m.name}. Now I use ids to get the name from the array and if I add more it'll automatically update.
When I use this helper method in a loop in the view, I think it will make multiple queries to get the Klass each time which means a lot of extra work.
I was wondering how I can "cache" this array or if I should be doing this a better way.
Thanks!
SQL caching is done automatically if you're within the same action. You can see here for a more detailed explanation. Just by the way, it would probably be more efficient to use pluck, as in Klass.pluck(:name). This would optimize your SQL query.
Your helper method should look similar to this
def klass_names
#klass_names ||= Klass.all.map{|m| m.name}
end
Following Problem:
I need something like an empty scope. Which means that this scope is emtpy, but responds to all methods a scope usually responds to.
I'm currently using a little dirty hack. I simply supply "1=0" as conditions. I find this realy ugly, since it hits the database. Simply returning an empty array won't work, since the result must respond to the scoped methods.
Is there a better existing solution for this or will I need to code this myself?
Maybe some example code could help explain what i need:
class User < ActiveRecord::Base
named_scope :admins, :conditions => {:admin => true }
named_scope :none_dirty, :conditions => "1=0" # this scope is always empty
def none_broken
[]
end
def self.sum_score # okay, a bit simple, but a method like this should work!
total = 0
self.all.each do |user|
total += user.score
end
return total
end
end
User.admin.sum_score # the score i want to know
User.none_drity.sum_score # works, but hits the db
User.none_broken.sum_score # ...error, since it doesn't respond to sum_score
Rails 4 introduces the none scope.
It is to be used in instances where you have a method which returns a relation, but there is a condition in which you do not want the database to be queried.
If you want a scope to return an unaltered scope use all:
No longer will a call to Model.all execute a query immediately and return an array of records. In Rails 4, calls to Model.all is equivalent to now deprecated Model.scoped. This means that more relations can be chained to Model.all and the result will be lazily evaluated.
User.where('false')
returns an ActiveRecord::Relation with zero elements, that is a chain-able scope that won't hit the database until you actually try to access one of its elements. This is similar to PhilT's solution with ('1=0') but a little more elegant.
Sorry User.scoped is not what you want. As commented this returns everything. Should have paid more attention to the question.
I've seen where('1 = 0') suggested before and Rails should probably cache it as well.
Also, where('1 = 0') won't hit the database until you do .all, .each, or one of the calculations methods.
I thing you need User.scoped({})
How about User.where(id: nil) ?
Or User.where(_id: nil) for mongoid.
The thing you are looking for does not exist. You could implement something like this by monky patching the find method. Yet, this would be an overkill, so I recomend keeping this unless it's performance critical.
Looking at your example code indicates you may not know about aggregated queries in SQL which are exposed as calculations methods in Rails:
User.sum(:score) will give you the sum of all users' scores
Take a look at Rails Guides for more info:
http://guides.rubyonrails.org/active_record_querying.html#sum
I have a simple find statement as such:
m = MyModel.find(1, :include => :my_children)
With m.mychildren being an Array; is there anyway to find a particular record from within the array without having to iterate over the entire thing. If I do mychildren.find(1), a new DB query is issues, which doesn't make sense, since they are all loaded already
It looks like there's a little Rails magic going on here. Where Enumerable#find is being overridden by ActiveRecord::Base#find on methods created for associations.
On the upside Enumerable#find is aliased to Enumerable#detect.
Unfortunately Enumerable#find/Enumerable#detect have significantly different syntax from ActiveRecord::Base#find.
So you can't just do mychildren.find(1), instead you've got to do mychildren.detect{|c| c.id == 1} if you want to avoid hitting the database again. You may also want to consider extending Array for a more DRY way of doing this.
class Array
def id_find id
self.detect{|element| element.id == id}
end
end
I'm not quite sure what your asking, but have you tried select:
m.mychildren.select{ |child| child == <<some_statement>> }
This won't hit the database assuming you've used the :include option as you stated in your question.
Alternatively, if you know the number of the child you want, you should be able to just use
m.mychildren[1]