SQL:
select * from user where room_id not in (select id from rooms);
what is the same equivalent query in rails console with this sql?
ex:
User.all.each { |u| user.room }
(sorry, but this example is not correct.)
You can translate it almost literally:
User.where('room_id not in (select id from rooms)').all
The where clause is quite flexible in what it accepts.
User.where("room_id not in (select id from rooms)")
but you want this since it would be rather faster:
User.where("not exist (select id from rooms where id=users.room_id)")
that's the closest you can get. There appears to be no way to create an Active Record query that translates to SQL NOT(). A search on the subject returns a bunch of SO questions with much the same answer.
You could do something like
User.all.select { |u| !Room.find_by_id(u.room_id) }
But that could be less efficient again.
I don't know if you are familiar with the squeel gem. It allows allows you to build very complex SQL-queries in a pure Ruby code. In your case it should be as simple as the following code (afer adding the gem 'squeel' line in your Gemfile and running bundle install):
room_ids = Room.select{id}; User.where{room_id.not_in room_ids}
Worth trying, isn't it?
Here's the squeel's page.
Related
I have a list of Products with a field called 'Title' and I have been trying to get a list of initial letters with not much luck. The closes I have is the following that dosn't work as 'Distinct' fails to work.
atoz = Product.objects.all().only('title').extra(select={'letter': "UPPER(SUBSTR(title,1,1))"}).distinct('letter')
I must be going wrong somewhere,
I hope someone can help.
You can get it in python after the queryset got in, which is trivial:
products = Project.objects.values_list('title', flat=True).distinct()
atoz = set([i[0] for i in products])
If you are using mysql, I found another answer useful, albeit using sql(django execute sql directly):
SELECT DISTINCT LEFT(title, 1) FROM product;
The best answer I could come up with, which isn't 100% ideal as it requires post processing is this.
atoz = sorted(set(Product.objects.all().extra(select={'letter': "UPPER(SUBSTR(title,1,1))"}).values_list('letter', flat=True)))
So I have an table of phone_numbers in Rails 3.2, and I would like to be able to make a query such as the following:
PhoneNumber.where("last_called_at - #{Time.now} > call_spacing * 60")
where last_called_at is a datetime and call_spacing is an integer, representing the number of minutes between calls.
I have tried the above, I have also tried using
PhoneNumber.where("datediff(mi, last_called_at, #{Time.now}) > call_spacing")
If anyone could help me make this work, or even recommend a current, up-to-date rails SQL gem, I would be very grateful.
Edit: So the best way to make this work for me was to instead add a ready_at column to my database, and update that whenever either call_spacing or last_called_at was updated, using the before_save method. This is probably the most semantic way to approach this problem in rails.
You have to use quotes around #{Time.now}. To make your first query work you may use TIME_TO_SEC() function:
PhoneNumber.where("(TIME_TO_SEC('#{Time.now}') - TIME_TO_SEC(last_called_at)) > (call_spacing * 60)")
Here is my way to do this:
PhoneNumber.where("last_called_at > '#{Time.zone.now.utc - (call_spacing * 60)}'")
also take a look at this article to be aware of how to play with timezones:
http://danilenko.org/2012/7/6/rails_timezones/
I have this use case where I get the symbolized deep associations from a certain model, and I have to perform certain queries that involve using outer joins. How can one do it WITHOUT resorting to write the full SQL by hand?
Answers I don't want:
- using includes (doesn't solve deep associations very well ( .includes(:cars => [:windows, :engine => [:ignition]..... works unexpectedly ) and I don't want its side-effects)
- writing the SQL myself (sorry, it's 2013, cross-db support, etc etc..., and the objects I fetch are read_only, more side-effects)
I'd like to have an Arel solution. I know that using the arel_table's from the models I can construct SQL expressions, there's also a DSL for the joins, but somehow i cannot use it in the joins method from the model:
car = Car.arel_table
engine = Engine.arel_table
eng_exp = car.join(engine).on(car[:engine_id].eq(engine[:id]))
eng_exp.to_sql #=> GOOD! very nice!
Car.joins(eng_exp) #=> Breaks!!
Why this doesn't work is beyond me. I don't know exactly what is missing. But it's the closest thing to a solution I have now. If somebody could help me completing my example or provide me with a nice work-around or tell me when will Rails include such an obviously necessary feature will have my everlasting gratitude.
This is an old question, but for the benefit of anyone finding it through search engines:
If you want something you can pass into .joins, you can either use .create_join and .create_on:
join_on = car.create_on(car[:engine_id].eq(engine[:id]))
eng_join = car.create_join(engine, join_on, Arel::Nodes::OuterJoin)
Car.joins(eng_join)
OR
use the .join_sources from your constructed join object:
eng_exp = car.join(engine, Arel::Nodes::OuterJoin).on(car[:engine_id].eq(engine[:id]))
Car.joins(eng_exp.join_sources)
I found a blog post that purports to address this problem: http://blog.donwilson.net/2011/11/constructing-a-less-than-simple-query-with-rails-and-arel/
Based on this (and my own testing), the following should work for your situation:
car = Car.arel_table
engine = Engine.arel_table
sql = car.project(car[Arel.star])
.join(engine, Arel::Nodes::OuterJoin).on(car[:engine_id].eq(engine[:id]))
Car.find_by_sql(sql)
If you don't mind adding a dependency and skipping AREL altogether, you could use Ernie Miller's excellent Squeel gem. It would be something like
Car.joins{engine.outer}.where(...)
This would require that the Car model be associated with Engine like so:
belongs_to :engine
I'm writing a Rails 3 ActiveRecord query using the "where" syntax, that uses both the SQL IN and the SQL OR operator and can't figure out how to use both of them together.
This code works (in my User model):
Question.where(:user_id => self.friends.ids)
#note: self.friends.ids returns an array of integers
but this code
Question.where(:user_id => self.friends.ids OR :target => self.friends.usernames)
returns this error
syntax error, unexpected tCONSTANT, expecting ')'
...user_id => self.friends.ids OR :target => self.friends.usern...
Any idea how to write this in Rails, or just what the raw SQL query should be?
You don't need to use raw SQL, just provide the pattern as a string, and add named parameters:
Question.where('user_id in (:ids) or target in (:usernames)',
:ids => self.friends.ids, :usernames => self.friends.usernames)
Or positional parameters:
Question.where('user_id in (?) or target in (?)',
self.friends.ids, self.friends.usernames)
You can also use the excellent Squeel gem, as #erroric pointed out on his answer (the my { } block is only needed if you need access to self or instance variables):
Question.where { user_id.in(my { self.friends.ids }) |
target.in(my { self.friends.usernames }) }
Though Rails 3 AR doesn't give you an or operator you can still achieve the same result without going all the way down to SQL and use Arel directly. By that I mean that you can do it like this:
t = Question.arel_table
Question.where(t[:user_id].in(self.friends.ids).or(t[:username].in(self.friends.usernames)))
Some might say it ain't so pretty, some might say it's pretty simply because it includes no SQL. Anyhow it most certainly could be prettier and there's a gem for it too: MetaWhere
For more info see this railscast: http://railscasts.com/episodes/215-advanced-queries-in-rails-3
and MetaWhere site: http://metautonomo.us/projects/metawhere/
UPDATE: Later Ryan Bates has made another railscast about metawhere and metasearch: http://railscasts.com/episodes/251-metawhere-metasearch
Later though Metawhere (and search) have become more or less legacy gems. I.e. they don't even work with Rails 3.1. The author felt they (Metawhere and search) needed drastic rewrite. So much that he actually went for a new gem all together. The successor of Metawhere is Squeel. Read more about the authors announcement here:
http://erniemiller.org/2011/08/31/rails-3-1-and-the-future-of-metawhere-and-metasearch/
and check out the project home page:
http://erniemiller.org/projects/squeel/
"Metasearch 2.0" is called Ransack and you can read something about it from here:
http://erniemiller.org/2011/04/01/ransack-the-library-formerly-known-as-metasearch-2-0/
Alternatively, you could use Squeel. To my eyes, it is simpler. You can accomplish both the IN (>>) and OR (|) operations using the following syntax:
Question.where{(:user_id >> my{friends.id}) | (:target >> my{friends.usernames})}
I generally wrap my conditions in (...) to ensure the appropriate order of operation - both the INs happen before the OR.
The my{...} block executes methods from the self context as defined before the Squeel call - in this case Question. Inside of the Squeel block, self refers to a Squeel object and not the Question object (see the Squeel Readme for more). You get around this by using the my{...} wrapper to restore the original context.
raw SQL
SELECT *
FROM table
WHERE user_id in (LIST OF friend.ids) OR target in (LIST OF friends.usernames)
with each list comma separate. I don't know the Rails ActiveRecord stuff that well. For AND you would just put a comma between those two conditions, but idk about OR
I was wondering if there was a way to use "find_by_sql" within a named_scope. I'd like to treat custom sql as named_scope so I can chain it to my existing named_scopes. It would also be good for optimizing a sql snippet I use frequently.
While you can put any SQL you like in the conditions of a named scope, if you then call find_by_sql then the 'scopes' get thrown away.
Given:
class Item
# Anything you can put in an sql WHERE you can put here
named_scope :mine, :conditions=>'user_id = 12345 and IS_A_NINJA() = 1'
end
This works (it just sticks the SQL string in there - if you have more than one they get joined with AND)
Item.mine.find :all
=> SELECT * FROM items WHERE ('user_id' = 887 and IS_A_NINJA() = 1)
However, this doesn't
Items.mine.find_by_sql 'select * from items limit 1'
=> select * from items limit 1
So the answer is "No". If you think about what has to happen behind the scenes then this makes a lot of sense. In order to build the SQL rails has to know how it fits together.
When you create normal queries, the select, joins, conditions, etc are all broken up into distinct pieces. Rails knows that it can add things to the conditions without affecting everything else (which is how with_scope and named_scope work).
With find_by_sql however, you just give rails a big string. It doesn't know what goes where, so it's not safe for it to go in and add the things it would need to add for the scopes to work.
This doesn't address exactly what you asked about, but you might investigate 'contruct_finder_sql'. It lets you can get the SQL of a named scope.
named_scope :mine, :conditions=>'user_id = 12345 and IS_A_NINJA() = 1'
named_scope :additional {
:condtions => mine.send(:construct_finder_sql,{}) + " additional = 'foo'"
}
sure why not
:named_scope :conditions => [ your sql ]