Don't compare datetimes with date ranges in MySQL and PostgreSQL

When selecting records in a date range, take care not to do it like this:

Copystart_date = Date.parse('2007-05-01')
end_date = Date.parse('2007-05-31')
LogItem.where(:created_at => start_date .. end_date)

The problem is that created_at is a datetime (or Time in Ruby), while start_date and end_date are simple dates. In order to make sense of your query, your database will cast your dates to datetimes where the time component is 00:00:00. Because of this the query above will lose records created from 2007-05-31 00:00:01 to 2007-05-31 23:59:59.

Details

The Ruby code above triggers the following query:

CopySELECT * FROM log_items WHERE created_at BETWEEN '2007-05-01' AND '2007-05-31'

Your database sees that you are trying to compare a datetime with a date and silently casts your query to:

CopySELECT * FROM log_items WHERE created_at BETWEEN '2007-05-01 00:00:00' AND '2007-05-31 00:00:00'

Note how you just lost all records from 2007-05-31 00:00:01 to 2007-05-31 23:59:59.

Solutions

One solution is to cast the datetime to a date before you make the query:

CopyLogItem.where('DATE(log_items.created_at) BETWEEN ? AND ?', start_date, end_date)

This may cause an index on the column to no longer be used.

Another solution is to be more explicit about what you want to select:

CopyLogItem.where(:created_at => start_date.beginning_of_day..end_date.end_of_day)