/activerecord/lib/active_record/relation/calculations.rb

require 'active_support/core_ext/object/blank'
require 'active_support/core_ext/object/try'

module ActiveRecord
  module Calculations
    # Count the records.
    #
    #   Person.count
    #   # => the total count of all people
    #
    #   Person.count(:age)
    #   # => returns the total count of all people whose age is present in database
    #
    #   Person.count(:all)
    #   # => performs a COUNT(*) (:all is an alias for '*')
    #
    #   Person.count(:age, distinct: true)
    #   # => counts the number of different age values
    def count(column_name = nil, options = {})
      column_name, options = nil, column_name if column_name.is_a?(Hash)
      calculate(:count, column_name, options)
    end

    # Calculates the average value on a given column. Returns +nil+ if there's
    # no row. See +calculate+ for examples with options.
    #
    #   Person.average('age') # => 35.8
    def average(column_name, options = {})
      calculate(:average, column_name, options)
    end

    # Calculates the minimum value on a given column. The value is returned
    # with the same data type of the column, or +nil+ if there's no row. See
    # +calculate+ for examples with options.
    #
    #   Person.minimum('age') # => 7
    def minimum(column_name, options = {})
      calculate(:minimum, column_name, options)
    end

    # Calculates the maximum value on a given column. The value is returned
    # with the same data type of the column, or +nil+ if there's no row. See
    # +calculate+ for examples with options.
    #
    #   Person.maximum('age') # => 93
    def maximum(column_name, options = {})
      calculate(:maximum, column_name, options)
    end

    # Calculates the sum of values on a given column. The value is returned
    # with the same data type of the column, 0 if there's no row. See
    # +calculate+ for examples with options.
    #
    #   Person.sum('age') # => 4562
    def sum(*args)
      if block_given?
        self.to_a.sum(*args) {|*block_args| yield(*block_args)}
      else
        calculate(:sum, *args)
      end
    end

    # This calculates aggregate values in the given column. Methods for count, sum, average,
    # minimum, and maximum have been added as shortcuts.
    #
    # There are two basic forms of output:
    #
    #   * Single aggregate value: The single value is type cast to Fixnum for COUNT, Float
    #     for AVG, and the given column's type for everything else.
    #
    #   * Grouped values: This returns an ordered hash of the values and groups them. It
    #     takes either a column name, or the name of a belongs_to association.
    #
    #       values = Person.group('last_name').maximum(:age)
    #       puts values["Drake"]
    #       => 43
    #
    #       drake  = Family.find_by_last_name('Drake')
    #       values = Person.group(:family).maximum(:age) # Person belongs_to :family
    #       puts values[drake]
    #       => 43
    #
    #       values.each do |family, max_age|
    #       ...
    #       end
    #
    # Examples:
    #   Person.calculate(:count, :all) # The same as Person.count
    #   Person.average(:age) # SELECT AVG(age) FROM people...
    #
    #   # Selects the minimum age for any family without any minors
    #   Person.group(:last_name).having("min(age) > 17").minimum(:age)
    #
    #   Person.sum("2 * age")
    def calculate(operation, column_name, options = {})
      relation = with_default_scope

      if relation.equal?(self)
        if eager_loading? || (includes_values.present? && references_eager_loaded_tables?)
          construct_relation_for_association_calculations.calculate(operation, column_name, options)
        else
          perform_calculation(operation, column_name, options)
        end
      else
        relation.calculate(operation, column_name, options)
      end
    rescue ThrowResult
      0
    end

    # This method is designed to perform select by a single column as direct SQL query
    # Returns <tt>Array</tt> with values of the specified column name
    # The values has same data type as column.
    #
    # Examples:
    #
    #   Person.pluck(:id) # SELECT people.id FROM people
    #   Person.uniq.pluck(:role) # SELECT DISTINCT role FROM people
    #   Person.where(:age => 21).limit(5).pluck(:id) # SELECT people.id FROM people WHERE people.age = 21 LIMIT 5
    #
    def pluck(column_name)
      key = column_name.to_s.split('.', 2).last

      if column_name.is_a?(Symbol) && column_names.include?(column_name.to_s)
        column_name = "#{table_name}.#{column_name}"
      end

      result = klass.connection.select_all(select(column_name).arel, nil, bind_values)
      types  = result.column_types.merge klass.column_types
      column = types[key]

      result.map do |attributes|
        value = klass.initialize_attributes(attributes)[key]
        if column
          column.type_cast value
        else
          value
        end
      end
    end

    private

    def perform_calculation(operation, column_name, options = {})
      operation = operation.to_s.downcase

      distinct = options[:distinct]

      if operation == "count"
        column_name ||= (select_for_count || :all)

        unless arel.ast.grep(Arel::Nodes::OuterJoin).empty?
          distinct = true
        end

        column_name = primary_key if column_name == :all && distinct

        distinct = nil if column_name =~ /\s*DISTINCT\s+/i
      end

      if group_values.any?
        execute_grouped_calculation(operation, column_name, distinct)
      else
        execute_simple_calculation(operation, column_name, distinct)
      end
    end

    def aggregate_column(column_name)
      if @klass.column_names.include?(column_name.to_s)
        Arel::Attribute.new(@klass.unscoped.table, column_name)
      else
        Arel.sql(column_name == :all ? "*" : column_name.to_s)
      end
    end

    def operation_over_aggregate_column(column, operation, distinct)
      operation == 'count' ? column.count(distinct) : column.send(operation)
    end

    def execute_simple_calculation(operation, column_name, distinct) #:nodoc:
      # Postgresql doesn't like ORDER BY when there are no GROUP BY
      relation = reorder(nil)

      if operation == "count" && (relation.limit_value || relation.offset_value)
        # Shortcut when limit is zero.
        return 0 if relation.limit_value == 0

        query_builder = build_count_subquery(relation, column_name, distinct)
      else
        column = aggregate_column(column_name)

        select_value = operation_over_aggregate_column(column, operation, distinct)

        relation.select_values = [select_value]

        query_builder = relation.arel
      end

      result = @klass.connection.select_value(query_builder, nil, relation.bind_values)
      type_cast_calculated_value(result, column_for(column_name), operation)
    end

    def execute_grouped_calculation(operation, column_name, distinct) #:nodoc:
      group_attr      = group_values
      association     = @klass.reflect_on_association(group_attr.first.to_sym)
      associated      = group_attr.size == 1 && association && association.macro == :belongs_to # only count belongs_to associations
      group_fields  = Array(associated ? association.foreign_key : group_attr)
      group_aliases = group_fields.map { |field| column_alias_for(field) }
      group_columns = group_aliases.zip(group_fields).map { |aliaz,field|
        [aliaz, column_for(field)]
      }

      group = @klass.connection.adapter_name == 'FrontBase' ? group_aliases : group_fields

      if operation == 'count' && column_name == :all
        aggregate_alias = 'count_all'
      else
        aggregate_alias = column_alias_for(operation, column_name)
      end

      select_values = [
        operation_over_aggregate_column(
          aggregate_column(column_name),
          operation,
          distinct).as(aggregate_alias)
      ]
      select_values += select_values unless having_values.empty?

      select_values.concat group_fields.zip(group_aliases).map { |field,aliaz|
        "#{field} AS #{aliaz}"
      }

      relation = except(:group).group(group.join(','))
      relation.select_values = select_values

      calculated_data = @klass.connection.select_all(relation, nil, bind_values)

      if association
        key_ids     = calculated_data.collect { |row| row[group_aliases.first] }
        key_records = association.klass.base_class.find(key_ids)
        key_records = Hash[key_records.map { |r| [r.id, r] }]
      end

      Hash[calculated_data.map do |row|
        key   = group_columns.map { |aliaz, column|
          type_cast_calculated_value(row[aliaz], column)
        }
        key   = key.first if key.size == 1
        key = key_records[key] if associated
        [key, type_cast_calculated_value(row[aggregate_alias], column_for(column_name), operation)]
      end]
    end

    # Converts the given keys to the value that the database adapter returns as
    # a usable column name:
    #
    #   column_alias_for("users.id")                 # => "users_id"
    #   column_alias_for("sum(id)")                  # => "sum_id"
    #   column_alias_for("count(distinct users.id)") # => "count_distinct_users_id"
    #   column_alias_for("count(*)")                 # => "count_all"
    #   column_alias_for("count", "id")              # => "count_id"
    def column_alias_for(*keys)
      table_name = keys.join(' ')
      table_name.downcase!
      table_name.gsub!(/\*/, 'all')
      table_name.gsub!(/\W+/, ' ')
      table_name.strip!
      table_name.gsub!(/ +/, '_')

      @klass.connection.table_alias_for(table_name)
    end

    def column_for(field)
      field_name = field.to_s.split('.').last
      @klass.columns.detect { |c| c.name.to_s == field_name }
    end

    def type_cast_calculated_value(value, column, operation = nil)
      case operation
        when 'count'   then value.to_i
        when 'sum'     then type_cast_using_column(value || '0', column)
        when 'average' then value.respond_to?(:to_d) ? value.to_d : value
        else type_cast_using_column(value, column)
      end
    end

    def type_cast_using_column(value, column)
      column ? column.type_cast(value) : value
    end

    def select_for_count
      if select_values.present?
        select = select_values.join(", ")
        select if select !~ /[,*]/
      end
    end

    def build_count_subquery(relation, column_name, distinct)
      column_alias = Arel.sql('count_column')
      subquery_alias = Arel.sql('subquery_for_count')

      aliased_column = aggregate_column(column_name == :all ? 1 : column_name).as(column_alias)
      relation.select_values = [aliased_column]
      subquery = relation.arel.as(subquery_alias)

      sm = Arel::SelectManager.new relation.engine
      select_value = operation_over_aggregate_column(column_alias, 'count', distinct)
      sm.project(select_value).from(subquery)
    end
  end
end