Optimize SQL query that retrieves count of items over a period - sql

Question

Is there a way to better optimize the query in this function? I am wanting it to only make a single SQL query if possible. The code picks up the number of events generated over 2 weeks. Thanks.
def items_chart_data
#current_student = Student.find(current_user.student_id)
(2.weeks.ago.to_date..DateTime.now).map do |date|
{
created_at: date,
item_count: Item.where("date(created_at) = ? AND student_id = ?", date, #current_student.id).count
}
end
end

Answer 1

You can do it in one query, like this:
items = Item.select('date(created_at) as date_created, count(id) as id_count').
where('student_id = ? and created_at >= ?', current_user.student_id, 2.weeks.ago.beginning_of_day).
group('date(created_at)').map do |item|
{ created_at: item.date_created, item_count: item.id_count }
end
To get the missing dates with 0 items into your array you can do this:
(2.weeks.ago.to_date..Date.current).each do |date|
date = date.strftime('%Y-%m-%d')
unless items.any? { |h| h.value?(date) } # Check if date exists already
items << { created_at: date, item_count: 0 } # Add item_count: 0 if not
end
end
items.sort_by! { |h| h[:created_at] } # Put array in correct order
Note that adding the zero item days to the array doesn't perform any queries. The array items contains only 14 elements, so (especially compared to querying the database 14 times) this code should be really fast.