Module: ActiveRecord::CounterCache

### Home Classes Methods

### In Files

- lib/active_record/counter_cache.rb

### Methods

- #decrement_counter
- #increment_counter
- #reset_counters
- #update_counters

### Files

- README.rdoc

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# ActiveRecord::CounterCache

# Active Record Counter Cache

### Public Instance Methods

decrement_counter(counter_name, id) click to toggle source

Decrement a number field by one, usually representing a count.

This works the same as increment_counter but reduces the column value by 1 instead of increasing it.

#### Parameters

- `counter_name` - The name of the field that should be decremented.

- `id` - The id of the object that should be decremented.

#### Examples

    # Decrement the post_count column for the record with an id of 5
    DiscussionBoard.decrement_counter(:post_count, 5)

    # File lib/active_record/counter_cache.rb, line 111
    def decrement_counter(counter_name, id)
      update_counters(id, counter_name => -1)
    end

increment_counter(counter_name, id) click to toggle source

Increment a number field by one, usually representing a count.

This is used for caching aggregate values, so that they don’t need to be computed every time. For example, a DiscussionBoard may cache post_count and comment_count otherwise every time the board is shown it would have to run an SQL query to find how many posts and comments there are.

#### Parameters

- `counter_name` - The name of the field that should be incremented.

- `id` - The id of the object that should be incremented.

#### Examples

    # Increment the post_count column for the record with an id of 5
    DiscussionBoard.increment_counter(:post_count, 5)

    # File lib/active_record/counter_cache.rb, line 94
    def increment_counter(counter_name, id)
      update_counters(id, counter_name => 1)
    end

reset_counters(id, *counters) click to toggle source

Resets one or more counter caches to their correct value using an SQL count query. This is useful when adding new counter caches, or if the counter has been corrupted or modified directly by SQL.

#### Parameters

- `id` - The id of the object you wish to reset a counter on.

- `counters` - One or more counter names to reset

#### Examples

    # For Post with id #1 records reset the comments_count
    Post.reset_counters(1, :comments)

    # File lib/active_record/counter_cache.rb, line 17
    def reset_counters(id, *counters)
      object = find(id)
      counters.each do |association|
        has_many_association = reflect_on_association(association.to_sym)

        expected_name = if has_many_association.options[:as]
          has_many_association.options[:as].to_s.classify
        else
          self.name
        end

        child_class  = has_many_association.klass
        belongs_to   = child_class.reflect_on_all_associations(:belongs_to)
        reflection   = belongs_to.find { |e| e.class_name == expected_name }
        counter_name = reflection.counter_cache_column

        self.unscoped.where(arel_table[self.primary_key].eq(object.id)).arel.update({
          arel_table[counter_name] => object.send(association).count
        })
      end
      return true
    end

update_counters(id, counters) click to toggle source

A generic “counter updater” implementation, intended primarily to be used by increment_counter and decrement_counter, but which may also be useful on its own. It simply does a direct SQL update for the record with the given ID, altering the given hash of counters by the amount given by the corresponding value:

#### Parameters

- `id` - The id of the object you wish to update a counter on or an Array of ids.

- `counters` - An Array of Hashes containing the names of the fields to update as keys and the amount to update the field by as values.

#### Examples

    # For the Post with id of 5, decrement the comment_count by 1, and
    # increment the action_count by 1
    Post.update_counters 5, :comment_count => -1, :action_count => 1
    # Executes the following SQL:
    # UPDATE posts
    #    SET comment_count = comment_count - 1,
    #        action_count = action_count + 1
    #  WHERE id = 5

    # For the Posts with id of 10 and 15, increment the comment_count by 1
    Post.update_counters [10, 15], :comment_count => 1
    # Executes the following SQL:
    # UPDATE posts
    #    SET comment_count = comment_count + 1,
    #  WHERE id IN (10, 15)

    # File lib/active_record/counter_cache.rb, line 69
    def update_counters(id, counters)
      updates = counters.map do |counter_name, value|
        operator = value < 0 ? '-' : '+'
        quoted_column = connection.quote_column_name(counter_name)
        "#{quoted_column} = COALESCE(#{quoted_column}, 0) #{operator} #{value.abs}"
      end

      update_all(updates.join(', '), primary_key => id )
    end

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