Question

In my SQLite-DB I want to select all entries on a monday.

How can I do it with Sequel?

A test example:

require 'sequel'

DB = Sequel.sqlite()#'test.db')
DB.create_table(:days) do
    String :text
    Date  :start
end

Date.today.upto(Date.today + 30){|d|
  DB[:days].insert( :text => d.strftime("%Y-%m-%d day %w in week %W"), :start => d)
}

How can I select all mondays?

In native SQL I can do:

select * from days where strftime("%w", start) = "1"

Using this, I can define a view and select for it:

DB.run('create view mondays as select * from days where strftime("%w", start) = "1"')
p DB[:mondays].all

But I would like to use it from Sequel.

I tried

sel = DB[:days].filter{ start.strftime("%w", :start) == '1' }  
#NoMethodError

sel = DB[:days].filter{ Sequel::SQL::Function.new(:strftime, "%w", :start) == '1' }
#SELECT * FROM `days` WHERE (1 = 0)

but without success.

Are there any other solutions?

I’m looking also for a possibility to select items by hour of day (all items with a timestamp, and then from 12:00-13:00 …) I think this is the same problem and a solution for the day of week-selection will also solve my other problem.

Answer 1

Here are the problems with your examples:

sel = DB[:days].filter{ start.strftime("%w", :start) == '1' }  
#NoMethodError

This uses a virtual row, so calling start returns a Sequel::SQL::Identifier. The Sequel::SQL::Identifier#strftime method does not exist, hence the NoMethodError.

sel = DB[:days].filter{ Sequel::SQL::Function.new(:strftime, "%w", :start) == '1' }
#SELECT * FROM `days` WHERE (1 = 0)

This fails because you are using ==. Sequel doesn’t override ==, and ruby’s default == returns false. On SQLite, Sequel represents false as (1 = 0).

This should work:

sel = DB[:days].filter{{strftime("%w", :start) => '1'}}
# SELECT * FROM `days` WHERE (strftime('%w', `start`) = '1')

This uses a hash (the inner {}) inside a virtual row block (the outer {}). In Sequel, equality is represented by hashes.