I have a Comment model which has-many attachments. What I want to return, is all of the comments which either have one or more attachment records, OR whose comment is longer than 250 characters.
Is there any way I can do this without writing it entirely in pure SQL? I'm struggling to build up a WHERE clause in just the rails method. It's not quite as simple as I'd hoped :(
Ideally I want this to be a scope but whatever will work is fine
You could try:
Comment.includes(:attachments).where('attachments.comment_id IS NOT NULL OR LEN(comments.content) > 250')
The WHERE clause should follow the pattern o the following pseudo-code
WHERE Length(Comment_field) > 250
OR EXISTS (Select COMMENT_ID from attachments)
Jump into the irb or rails c (console) do this from command-line to get it then plug it in.
c = YourCommentModel.where('attachments > ?', 1)
len250 = c = YourCommentModel.where('attachments.length> ?', 250)
first one gives comments of greater than 1, second gives comments > 250
Related
I have a Work model with a video_id, a user_id and some other simple fields. I need to display the last 12 works on the page, but only take 1 per user. Currently I'm trying to do it like this:
def self.latest_works_one_per_user(video_id=nil)
scope = self.includes(:user, :video)
scope = video_id ? scope.where(video_id: video_id) : scope.where.not(video_id: nil)
scope = scope.order(created_at: :desc)
user_ids = works = []
scope.each do |work|
next if user_ids.include? work.user_id
user_ids << work.user_id
works << work
break if works.size == 12
end
works
end
But I'm damn sure there is a more elegant and faster way of doing it especially when the number of works gets bigger.
Here's a solution that should work for any SQL database with minimal adjustment. Whether one thinks it's elegant or not depends on how much you enjoy SQL.
def self.latest_works_one_per_user(video_id=nil)
scope = includes(:user, :video)
scope = video_id ? scope.where(video_id: video_id) : scope.where.not(video_id: nil)
scope.
joins("join (select user_id, max(created_at) created_at
from works group by created at) most_recent
on works.user_id = most_recent.user_id and
works.created_at = most_recent.created_at").
order(created_at: :desc).limit(12)
end
It only works if the combination of user_id and created_at is unique, however. If that combination isn't unique you'll get more than 12 rows.
It can be done more simply in MySQL. The MySQL solution doesn't work in Postgres, and I don't know a better solution in Postgres, although I'm sure there is one.
I have 2 models - Restaurant and Feature. They are connected via has_and_belongs_to_many relationship. The gist of it is that you have restaurants with many features like delivery, pizza, sandwiches, salad bar, vegetarian option,… So now when the user wants to filter the restaurants and lets say he checks pizza and delivery, I want to display all the restaurants that have both features; pizza, delivery and maybe some more, but it HAS TO HAVE pizza AND delivery.
If I do a simple .where('features IN (?)', params[:features]) I (of course) get the restaurants that have either - so or pizza or delivery or both - which is not at all what I want.
My SQL/Rails knowledge is kinda limited since I'm new to this but I asked a friend and now I have this huuuge SQL that gets the job done:
Restaurant.find_by_sql(['SELECT restaurant_id FROM (
SELECT features_restaurants.*, ROW_NUMBER() OVER(PARTITION BY restaurants.id ORDER BY features.id) AS rn FROM restaurants
JOIN features_restaurants ON restaurants.id = features_restaurants.restaurant_id
JOIN features ON features_restaurants.feature_id = features.id
WHERE features.id in (?)
) t
WHERE rn = ?', params[:features], params[:features].count])
So my question is: is there a better - more Rails even - way of doing this? How would you do it?
Oh BTW I'm using Rails 4 on Heroku so it's a Postgres DB.
This is an example of a set-iwthin-sets query. I advocate solving these with group by and having, because this provides a general framework.
Here is how this works in your case:
select fr.restaurant_id
from features_restaurants fr join
features f
on fr.feature_id = f.feature_id
group by fr.restaurant_id
having sum(case when f.feature_name = 'pizza' then 1 else 0 end) > 0 and
sum(case when f.feature_name = 'delivery' then 1 else 0 end) > 0
Each condition in the having clause is counting for the presence of one of the features -- "pizza" and "delivery". If both features are present, then you get the restaurant_id.
How much data is in your features table? Is it just a table of ids and names?
If so, and you're willing to do a little denormalization, you can do this much more easily by encoding the features as a text array on restaurant.
With this scheme your queries boil down to
select * from restaurants where restaurants.features #> ARRAY['pizza', 'delivery']
If you want to maintain your features table because it contains useful data, you can store the array of feature ids on the restaurant and do a query like this:
select * from restaurants where restaurants.feature_ids #> ARRAY[5, 17]
If you don't know the ids up front, and want it all in one query, you should be able to do something along these lines:
select * from restaurants where restaurants.feature_ids #> (
select id from features where name in ('pizza', 'delivery')
) as matched_features
That last query might need some more consideration...
Anyways, I've actually got a pretty detailed article written up about Tagging in Postgres and ActiveRecord if you want some more details.
This is not "copy and paste" solution but if you consider following steps you will have fast working query.
index feature_name column (I'm assuming that column feature_id is indexed on both tables)
place each feature_name param in exists():
select fr.restaurant_id
from
features_restaurants fr
where
exists(select true from features f where fr.feature_id = f.feature_id and f.feature_name = 'pizza')
and
exists(select true from features f where fr.feature_id = f.feature_id and f.feature_name = 'delivery')
group by
fr.restaurant_id
Maybe you're looking at it backwards?
Maybe try merging the restaurants returned by each feature.
Simplified:
pizza_restaurants = Feature.find_by_name('pizza').restaurants
delivery_restaurants = Feature.find_by_name('delivery').restaurants
pizza_delivery_restaurants = pizza_restaurants & delivery_restaurants
Obviously, this is a single instance solution. But it illustrates the idea.
UPDATE
Here's a dynamic method to pull in all filters without writing SQL (i.e. the "Railsy" way)
def get_restaurants_by_feature_names(features)
# accepts an array of feature names
restaurants = Restaurant.all
features.each do |f|
feature_restaurants = Feature.find_by_name(f).restaurants
restaurants = feature_restaurants & restaurants
end
return restaurants
end
Since its an AND condition (the OR conditions get dicey with AREL). I reread your stated problem and ignoring the SQL. I think this is what you want.
# in Restaurant
has_many :features
# in Feature
has_many :restaurants
# this is a contrived example. you may be doing something like
# where(name: 'pizza'). I'm just making this condition up. You
# could also make this more DRY by just passing in the name if
# that's what you're doing.
def self.pizza
where(pizza: true)
end
def self.delivery
where(delivery: true)
end
# query
Restaurant.features.pizza.delivery
Basically you call the association with ".features" and then you use the self methods defined on features. Hopefully I didn't misunderstand the original problem.
Cheers!
Restaurant
.joins(:features)
.where(features: {name: ['pizza','delivery']})
.group(:id)
.having('count(features.name) = ?', 2)
This seems to work for me. I tried it with SQLite though.
I have a User that has_many messages.
I need a create a query that will
'Get me all users who's (message.opened == false) count < 3'
Right now, I am using User.all, iterating through all users, and counting manually. I understand that this isn't very efficient and it can be all done in one query, but I am new to SQL/ActiveRecord so need some help here.
Thanks
Assuming Rails 3 syntax. You can do something like:
User.joins(:messages).where(:messages => {:opened => false}).group(:user_id).having("COUNT(messages.id) < 3)
This should work:
User.includes(:messages).group("users.id").where("messages.opened = 0").having("count(messages.id) < 3")
This will create two queries, one for the grouped query, and one for eager loading the resulting users and messages with a join.
Here is solution to your problem
User.includes(:messages).group("users.id").where("messages.opened = 0").having("count(messages.id) < 3")
but what else you can do is to create a scope for this
scope :not_opened_less_three_count, includes(:messages).group("users.id").where("messages.opened = 0").having("count(messages.id) < 3")
And then you can use it anywhere you needed as follow
User.not_opened_less_three_count
Try this
User.includes(:messages).group('users.id').having('SUM(IFNULL(messages.opened = 0, 1)) < 3')
It works at least on MySQL, AND assuming your boolean true are 1 in database.
EDIT I had reversed the condition
PS IFNULL is there to handle if messages.opened can be NULL
I'm doing a rails app. I have to do a comparison engine a bit complex. I'm currently trying to do a prototype. My query can vary widely so i have to work with a lot of scopes, but that's not my problem.
My query have to compare candidates. These candidates have answered some tests. These tests belongs to category. Theses tests have different max value, and i have to be able to compare candidates by categories.
So i have to calculate a % of good answers. I have to be able to compare candidates in all possible use cases in one category. So, i have to be able to compare the average good answer rate for all this category.
In a nutshell : I have to be able to use subqueries in order to compare some candidates. I have to be able to compare them for a test or a category. My problem is using a subquery able to return a good answer rate for all tests a candidats may have passed in a category.
And I have to be able to use this subquery in an order_by or having clause.
How can I construct this subquery ? I have no problem to handle complex conditional queries with some scopes. This has to be a real subquery, because I am working with 6 or 7 models here.
I ask for an active record way, cause this must work with whatever database supported by rails.
Excuse my poor English.
Edit :
An example is worth 1000 words so how could do something like this :
Sessiontest.find(Candidat.where(:firstname => 'toto'))
This example is stupid, ok. So, is it possible to do something like this ?
Edit2 :
I saw some posts about AREL. I wish to know if it is possible to do this without a third party plugin.
Is it possible to do some sub queries in subqueries with arel? Because for example, my number of points per test, is the sum of the points of all his questions. (Sad, but I have to keep it). And I need this, so my subquery can calculate my good answers %.
So you got the idea. That's something, which has to be really powerful, so I need something powerful, and not too much error prone.
Edit3 : I made some progress, but I can't for a while post an answer.
It seem possible to get this work without any plugin. I have some success in buildings some subqueries like this :
toto = Candidat.where(:lastname => Candidat.select(:lastname).where(:lastname => "ulysse").limit(1))
The request :
Candidat Load (1.0ms)[0m SELECT "candidats".* FROM "candidats" WHERE "candidats"."lastname" IN (SELECT "candidats"."id" FROM "candidats" WHERE "candidats"."lastname" = 'ulysse' LIMIT 1
This works and create a real subquery. I will try some more advanced experiences, in order to get the level I actually need.
Just tried sub-subquery works wonder too.
Edit 5 :
I am trying some more advanced things, and there is a lot of things, i still don't understand.
- toto = Candidat.where("id = ? / ? ", Sessiontest.select(:id).where(:id => 6), Sessiontest.select(:id).where(:id => 2))
This is just a stupid example in order to get an object with an id of 3. This code works, but not as i expected.
See, the sql :
1m[35m (1.0ms)[0m SELECT COUNT("sessiontests"."id") FROM "sessiontests" WHERE "sessiontests"."id" = 6
[1m[36mSessiontest Load (0.0ms)[0m [1mSELECT id FROM "sessiontests" WHERE "sessiontests"."id" = 6[0m
[1m[35m (1.0ms)[0m SELECT COUNT("sessiontests"."id") FROM "sessiontests" WHERE "sessiontests"."id" = 2
[1m[36mSessiontest Load (1.0ms)[0m [1mSELECT id FROM "sessiontests" WHERE "sessiontests"."id" = 2[0m
[1m[35mCandidat Load (1.0ms)[0m SELECT "candidats".* FROM "candidats" WHERE (id = 6 / 2)
So, it does not use a subqueries. I tried with .to_sql. But it introduce my sql this way :
1m[36mCandidat Load (0.0ms)[0m [1mSELECT "candidats".* FROM "candidats" WHERE (id = 'SELECT id FROM "sessiontests" WHERE "sessiontests"."id" = 6' / 2 )[0m
So active record quoted the subreust for security purpose. this is closer to my wish, but not really what i want.
This does not work
Candidat.where("id = (?) / ? ", Sessiontest.select(:id).where(:id => 6).to_sql, Sessiontest.select(:id).where(:id => 2))
Quotes prevents the subquery to work.
But this work :
Candidat.where("id = (" + Sessiontest.select(:id).where(:id => 6).to_sql + ") / (" + Sessiontest.select(:id).where(:id => 2).to_sql + ") ")
[1m[36mCandidat Load (1.0ms)[0m [1mSELECT "candidats".* FROM "candidats" WHERE (id = (SELECT id FROM "sessiontests" WHERE "sessiontests"."id" = 6) / (SELECT id FROM "sessiontests" WHERE "sessiontests"."id" = 2) )[0m
But I find this ugly. I will try to get these subqueries working in a more dynamic way. I mean replace the integer values by columns name.
I don't have anymore the exact answer to this question, because i do not work in the same enterprise anymore. But the solution to this problem, was to use a group_by clause. So the request became really easy.
With a group_by, i was able to manipulate, category or a technology with ease.
ActiveRecord objects of the class 'Location' (representing the db-table Locations) have the attributes 'url', 'lat' (latitude) and 'lng' (longitude).
Lat-lng-combinations on this model should be unique. The problem is, that there are a lot of Location-objects in the database having duplicate lat-lng-combinations.
I need help in doing the following
Find objects that share the same
lat-lng-combination.
If the 'url' attribute of the object
isn't empty, keep this object and delete the
other duplicates. Otherwise just choose the
oldest object (by checking the attribute
'created_at') and delete the other duplicates.
As this is a one-time-operation, solutions in SQL (MySQL 5.1 compatible) are welcome too.
If it's a one time thing then I'd just do it in Ruby and not worry too much about efficiency. I haven't tested this thoroughly, check the sorting and such to make sure it'll do exactly what you want before running this on your db :)
keep = []
locations = Location.find(:all)
locations.each do |loc|
# get all Locations's with the same coords as this one
same_coords = locations.select { |l| l.lat == loc.lat and \
l.lng == loc.lng }
with_urls = same_coords.select { |l| !l.url.empty? }
# decide which list to use depending if there were any urls
same_coords = with_urls.any? ? with_urls : same_coords
# pick the best one
keep << same_coords.sort { |a,b| b.created_at <=> a.created_at }.first.id
end
# only keep unique ids
keep.uniq!
# now we just delete all the rows we didn't decide to keep
locations.each do |loc|
loc.destroy unless keep.include?( loc.id )
end
Now like I said, this is definitely poor, poor code. But sometimes just hacking out the thing that works is worth the time saved in thinking up something 'better', especially if it's just a one-off.
If you have 2 MySQL columns, you can use the CONCAT function.
SELECT * FROM table1 GROUP BY CONCAT(column_lat, column_lng)
If you need to know the total
SELECT COUNT(*) AS total FROM table1 GROUP BY CONCAT(column_lat, column_lng)
Or, you can combine both
SELECT COUNT(*) AS total, table1.* FROM table1
GROUP BY CONCAT(column_lat, column_lng)
But if you can explain more on your question, perhaps we can have more relevant answers.