Rails中使用ActiveRecord查询上月支出时无法获取31号数据的问题

Let's break down why you're missing the 31st day's data and how to fix it:

The Root Cause #

When you use Date objects to query a datetime column (like created_at), Rails automatically converts the Date to a DateTime at midnight (00:00:00) of that date.

For example, if you're querying August 2020 in September, (Date.today.end_of_month) -1.month gives you the Date 2020-08-31, but it gets converted to 2020-08-31 00:00:00 in the query. That means your range only includes records up to August 30th 23:59:59 — any entries created on August 31st (with a time after midnight) fall outside the range.

Solution 1: Explicitly Cover the Full Last Day #

Instead of relying on relative date math that cuts off the 31st, explicitly define the start and end of the previous month with full datetime coverage:

# Get the full datetime range for the previous month
prev_month_start = Date.today.prev_month.beginning_of_month.beginning_of_day
prev_month_end = Date.today.prev_month.end_of_month.end_of_day

# Run the query with the complete range
Expend.where(cc: "PUQ").where(created_at: prev_month_start..prev_month_end).sum(:total)

Solution 2: Use Rails' Built-in all_month Helper #

Rails has a shortcut to handle this exact scenario. The all_month method returns a range from the first second of the month's start to the last second of the month's end, ensuring no days get cut off:

Expend.where(cc: "PUQ").where(created_at: Date.today.prev_month.all_month).sum(:total)

Why Your Original Code Failed #

Let's unpack your original line for August 2020 (assuming today was September 2020):

(Date.today.beginning_of_month)-1.month # => 2020-08-01 (Date object)
(Date.today.end_of_month)-1.month # => 2020-08-31 (Date object)

When converted to datetime for the database query, this becomes 2020-08-01 00:00:00 to 2020-08-31 00:00:00. Any created_at timestamp on August 31st (like 2020-08-31 14:30:00) is after the end of your range, so it gets excluded.

Using end_of_day or all_month extends the range to 2020-08-31 23:59:59.999999, which captures every record from that day.

内容的提问来源于stack exchange，提问作者Poluxland