Ruby on Rails - Activerecord::Relation Count Method Is Wrong

Ruby on Rails - ActiveRecord::Relation count method is wrong?

see this Rails 3: Difference between Relation.count and Relation.all.count

In short Rails ignores the select columns (if more than one) when you apply count to the query. This is because

SQL's COUNT allows only one or less columns as parameters.

From Mailbox code

 scope :participant, lambda {|participant|
    select('DISTINCT conversations.*').
      where('notifications.type'=> Message.name).
      order("conversations.updated_at DESC").
      joins(:receipts).merge(Receipt.recipient(participant))
  }

self.mailbox.conversations.count ignores the select('DISTINCT conversations.*') and counts the join table with receipts, essentially counting number of receipts with duplicate conversations in it.

On the other hand, self.mailbox.conversations.all.count first gets the records applying the select, which gets unique conversations and then counts it.

self.mailbox.conversations.all == self.mailbox.conversations since both of them query the db with the select.

To solve your problem you can use sending_user.mailbox.conversations.all.count or sending_user.mailbox.conversations.group('conversations.id').length

Is overriding an ActiveRecord relation's count() method okay?

Edit:

I checked the code of will_paginate, seems like it is not using count method of AR relation, but i found that you can provide option total_entries for paginate

@kids = @parent.kids.for_chatting.paginate(
  page:          params[:page], 
  total_entries: parent.chatty_kids_count
)

This is not working

You can use wrapper for collection like here
https://github.com/kaminari/kaminari/pull/818#issuecomment-252788488​,
just override count method.

class RelationWrapper < SimpleDelegator
  def initialize(relation, total_count)
    super(relation)
    @total_count = total_count
  end

  def count
    @total_count 
  end
end

# in a controller:
relation = RelationWrapper.new(@parent.kids.for_chatting, parent.chatty_kids_count)

Rails5.2 - Active Record .select() causing error when .count is called

 User.all.select('users.*,"one" as one').count(:all)

and

 User.all.select('users.*,"one" as one').count(:id)

Both work because it specifies what the SQL code should count on.

Rails 3: Difference between Relation.count and Relation.all.count

Okay, thanks to tadman for pushing me in the right direction.

I digged somewhat deeper (especially in the log files) and what i found is a little bit weird.

The problem was caused by the number of selected columns. If one selects only one column and counts the result

my_meal.noodles.select("distinct color").count

ActiveRecord creates the following SQL statement:

SELECT COUNT(distinct color) AS count_id FROM "NOODLES" WHERE ("NOODLES".meal_id = 295)

In case one selects two or more columns and applies count to it

my_meal.noodles.select("distinct color, shape").count

ActiveRecord forgets about that select clause and creates:

SELECT COUNT(*) AS count_id FROM "NOODLES" WHERE ("NOODLES".meal_id = 295)

This may be right, since (SQL's) COUNT allows only one or less columns as parameters. Add a group before the count and anything is fine:

my_meal.noodles.select("distinct color, shape").group("color, shape").count

SELECT COUNT(*) AS count_all, color, shape AS color_shape FROM "NOODLES" WHERE ("NOODLES".meal_id = 295) GROUP BY color, shape

Apart from this AS color_shape it is exact what i expected. BUT... only it returns this:

>> my_meal.noodles.select("distinct color, shape").group("color, shape").count
=> {star=>309, circle=>111, spaghetti=>189, square=>194, triangle=>179, bowtie=>301, shell=>93, letter=>230}

>> my_meal.noodles.select("distinct color, shape").group("color, shape").count.class
=> ActiveSupport::OrderedHash

This weird return value is (apart from order which depends on the DB) identical with the result and return value of

my_meal.noodles.group("shape").count

Conclusion:

As pointed out here there is still a gap between relations (may they be mathematical or arel relations) and ActiveRecord::Relations.

I can see the advantages of pressing the result in the patterns of a model as often as possible (at least in the context of a Rails app).

However real relations are not the result of the combination of several operations but the result of the concatenation of those operations.
In general the chainability of ActiveRecord::Relations is a great thing but there are some design decisions i cannot follow.

If you can't depend on the assurance that each action returns a new relation to work with, it looses much of its inuitional appeal.

As for the solution of my problem, i will use the above-mentioned group solution and some kind of dirty workaround for the count operation:

my_meal.noodles.select("distinct color, shape").group("color, shape").all.count

This compresses the results to an acceptable minimum before pulling them out of the database and creating expensive objects just to count them. Alternatively one could use a handwritten SQL query, but why have a Rails and do not use it, huh? ;-)

Thanks for your help,

Tim

Count Method on Undefined Active Record Array @objects.count

There are many ways to write something like this.

Something simple would be:

<% if @objects %>
  <%= @objects.count %>
<% else %>
  nothing found
<% end %>

If you get into a slightly more complex conditional I would suggest moving the logic into a helper and call it from the view. ex:

<%= count_for(@object) %>

query , can not select column count

Have you tried calling the count method on one of the returned Tag objects? Just because inspect doesn't mention the count doesn't mean that it isn't there. The inspect output:

[#<Tag id: 401, name: "different">, ... , #<Tag id: 4, name: "family">]

will only include things that the Tag class knows about and Tag will only know about the columns in the tags table: you only have id and name in the table so that's all you see.

If you do this:

tags = Tag.joins(:quote_tags).group('quote_tags.tag_id').order('count desc').select('count(tags.id) AS count, tags.id, tags.name')

and then look at the counts:

tags.map(&:count)

You'll see the array of counts that you're expecting.

Why does Relation.size sometimes return a Hash in Rails 4

size on an ActiveRecord::Relation object translates to count, because the former tries to get the count of the Relation. But when you call count on a grouped Relation object, you receive a hash.

The keys of this hash are the grouped column's values; the values of this hash are the respective counts.

AssessmentResponse.group(:client_id).count # this will return a Hash
AssessmentResponse.group(:client_id).size # this will also return a Hash

This is true for the following methods: count, sum, average, maximum, and minimum.

If you want to check for rows being present or not, simply use exists? i.e. do the following:

AssessmentResponse.group(:client_id).exists?

Instead of this:

AssessmentResponse.group(:client_id).count.zero?

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