在Ruby on Rails中减去数据表的2个字段

Question

I have the following problem.

I have a table lets call it Activity.

In this table there are 3 fields one is the date_start , the second is date_end and the third is the activity_type , which correspond the date that the activity start and ends and the type of activity for example physical activity and eating.

I would like to find the median time period that a activity_type lasts.

For example:

Activity.where(activity_type: "Eating healthy")

So I want to apply in this question, a sum statement with the time difference between the date_start and date_end .

How I can find the time differences of all the same type activities?

Answer 1

You can use SQL's DATEDIFF function to find the number of days between each date value. You can then use ActiveRecord 's average method to find out the median/average number of days.

condition = {activity_type: "Eating healthy"}
sql_string = "DATEDIFF(day, date_end, date_start)"
Activity.where(condition).average(sql_string) #=> numeric value

You can get the data for all activity types by grouping the rows, like so:

Activity.group(:activity_type).average(sql_string) #=> Hash with numeric values

But this also depends on which database you're using. For example, the DATEDIFF function doesn't exist for SQLite. You will have to lookup the appropriate date-difference function that you can use for your database.