I have the following setup:
class Season < AR::Base
has_many :date_ranges
end
class DateRange < AR::Base
# has a :starts_at & :ends_at
end
How would I find all overlapping seasons from a season instance? I have already tried with a couple of different queries (below). But the problem I keep hitting is the fact that the season im checking for also possible has multiple date_ranges. I could solve it with a loop but i'd rather only use a query.
This query looks up all the seasons that overlap but it only does that for 1 input date_range
Season.joins(:date_ranges).where("starts_at <= ? AND ends_at >= ?", ends_at, starts_at)
Maybe I need something to chain a couple of OR's together for each date_range on the instance but where() only uses AND.
So in short, finding the overlap is not the problem, but how do I find overlap of multiple date_ranges to the entire database?
The easiest way to do this is through straight SQL. Something like this:
DateRange.find_by_sql(%q{
select a.*
from date_ranges a
join date_ranges b on
a.id < b.id
and (
(a.ends_at >= b.starts_at and a.ends_at <= b.ends_at)
or (a.starts_at >= b.starts_at and a.starts_at <= b.ends_at)
or (a.starts_at <= b.starts_at and a.ends_at >= b.ends_at)
)
where season_id = ?
}, season_id)
The basic idea is to join the table to itself so that you can easily compare the ranges. The a.id < b.id is there to get unique results and filter out "ranges matches itself" cases. The inner or conditions check for both types of overlaps:
[as-----ae] [as-----ae]
[bs-----be] [bs-----be]
and
[as--------------ae] [as----ae]
[bs----be] [bs--------------be]
You might want to think about the end points though, that query considers two intervals to overlap if they only match at an endpoint and that might not be what you want.
Presumably you already have a unique constraint on the (season_id, starts_at, ends_at) triples and presumably you're already ensuring that starts_at <= ends_at.
Related
I would like to fetch all records per day with highest priority (as defined in associated model)
I'm struggling to build this with activerecord (rails 4.2)
The problem is very similar to this one
Get records with max value for each group of grouped SQL results
except that the age would come from the second model
or also this one
with activerecord how can I select records based on the highest value of a field?
Model 1: Workduration:
date, duration
belongs_to :timerule
Model 2: Timerule:
priority
has_many :workdurations
I put together the data as follows (all in Workduration)
def self.withPrio
select("workdurations.*, timerules.prio AS prio").joins(:timerule)
end
I couldn't find the proper way to build the LEFT OUTER JOIN (self-join) on it.
Try-And-Error-Code:
Workduration.withPrio.joins("left join ? workdurations.date = wd2.date and workdurations.prio < wd2.prio", Workduration.withPrio)
Any help is appreciated!
I ended up doing this with (a big) find_by_sql and a second query to keep the scopes chainable:
scope :maxPrioIds, ->{find_by_sql('SELECT o.*
FROM
(SELECT workdurations.*, timerules.prio AS prio FROM "workdurations" INNER JOIN "timerules" ON "timerules"."id" = "workdurations"."timerule_id") o
LEFT JOIN (SELECT workdurations.*, timerules.prio AS prio FROM "workdurations" INNER JOIN "timerules" ON "timerules"."id" = "workdurations"."timerule_id") b
ON o.date = b.date AND o.prio < b.prio
WHERE b.prio is NULL').map(&:id)}
scope :relevant, -> {where(id: Workduration.maxPrioIds)}
In my Rails App, I did a alot of range search to group objects, like
scope :best_of_the_week, ->(time) do
start_time = time.beginning_of_week
end_time = time.end_of_week
where("created_at > ? AND created_at < ?", start_time, end_time).where('votes_count > ?', 300).order('votes_count DESC').first(8)
end
In this case, do I need to add index to created_at? and what about votes_count?
Addtionally, how can I elegantly combine the first two where searches? Or does combining them make any difference?
If you want max performance to this query, create an index for both. If you don't want to create too many indexes, you should index created_at, date seems do have a bigger range as the time goes (and size of database).
I like to use the find_by_sql and make SELECT retrieve just the essential data to improve performance, if you have too many var chars fields this will have a nice impact.
Just for sintax sugar
where("between ? and ?", start_time, end_time).(other stuff)
I came across about the problem excluding data, if the attribute x of one of the associated data has the value 'a'.
Example:
class Order < ActiveRecord::Base
has_many :items
end
class Item < ActiveRecord::Base
belongs_to :order
validate_presence_of :status
end
The query should return all Orders that don't have an Item with status = 'paid' (status != 'paid').
Because of the 1:n association an Order can have many Items. And one of the Itmes can have the status = 'paid'. These Orders must be excluded from the result of my query even if the order has other items with status different from 'paid'.
How would I solve this problem:
paid_items = Items.where(status: 'paid').pluck(:order_id)
orders_wo_paid = Order.where('id NOT IN (?)', paid_items)
Is there an ActiveRecord solution, that solves this problem in one query.
Or are there other ways to solve this question?
I 'm not looking for ruby solution such as:
Order.select do |order|
!order.items.pluck(:status).include?('paid')
end
thx for ideas and inspirations.
You can do:
Order.where('orders.id NOT IN (?)', Item.where(status: 'paid').select(:order_id))
If you're using Rails 4.x then:
Order.where.not(id: Item.where(status: 'paid').select(:order_id))
The query you are interested in is the following, but creating with activerecord will be hard/no very readable:
SELECT
orders.*
FROM
orders
LEFT JOIN
order_items ON orders.id = order_items.order_id
GROUP BY
order_items.order_id
HAVING
COUNT(DISTINCT order_items.id) = COUNT(DISTINCT order_items.status <> 'paid')
Sorry for the sql indentation, I have no idea which are the conventions for it.
A way (not the best one at all) to it with rails (unfortunately writing sql for the most important parts) would be the following:
Order.group(:order_id).joins("LEFT JOIN order_items ON orders.id = order_items.order_id")
.having("COUNT(DISTINCT order_items.id) = COUNT(DISTINCT order_items.status <> 'paid')")
Of course you can play with AREL to get rid of the hard coded sql, but in my opinion it will not be easier to read.
You can have an example of creating lefts joins in this gist: https://gist.github.com/mildmojo/3724189
I'm building a search UI which searches for comments. When a user clicks on a search result (comment), I want to show the surrounding comments.
My model:
Group (id, title) - A Group has many comments
Comment (id, group_id, content)
For example:
When a user clicks on a comment with comment.id equal to 26. I would first find all the comments for that group:
comment = Comment.find(26)
comments = Comment.where(:group_id => comment.group_id)
I now have all of the group's comments. What I then want to do is show comment.id 26, with a max of 10 comments before and 10 comments after.
How can I modify comments to show that offset?
Sounds simple, but it's tricky to get the best performance for this. In any case, you must let the database do the work. That will be faster by an order of magnitude than fetching all rows and filter / sort on the client side.
If by "before" and "after" you mean smaller / bigger comment.id, and we further assume that there can be gaps in the id space, this one query should do all:
WITH x AS (SELECT id, group_id FROM comment WHERE id = 26) -- enter value once
(
SELECT *
FROM x
JOIN comment c USING (group_id)
WHERE c.id > x.id
ORDER BY c.id
LIMIT 10
)
UNION ALL
(
SELECT *
FROM x
JOIN comment c USING (group_id)
WHERE c.id < x.id
ORDER BY c.id DESC
LIMIT 10
)
I'll leave paraphrasing that in Ruby syntax to you, that's not my area of expertise.
Returns 10 earlier comments and 10 later ones. Fewer if fewer exist. Use <= in the 2nd leg of the UNION ALL query to include the selected comment itself.
If you need the rows sorted, add another query level on top with ORDER BY.
Should be very fast in combination with these two indexes for the table comment:
one on (id) - probably covered automatically the primary key.
one on (group_id, id)
For read-only data you could create a materialized view with a gap-less row-number that would make this even faster.
More explanation about parenthesis, indexes, and performance in this closely related answer.
Something like:
comment = Comment.find(26)
before_comments = Comment.
where('created_at <= ?', comment.created_at).
where('id != ?', comment.id).
where(group_id: comment.group_id).
order('created_at DESC').limit(10)
after_comments = Comment.
where('created_at >= ?', comment.created_at).
where('id != ?', comment.id).
where(group_id: comment.group_id).
order('created_at DESC').limit(10)
I have two models: ScheduledCourse and ScheduledSession.
scheduled_course has_many scheduled_sessions
scheduled_session belongs_to scheduled_course
ScheduledCourse has a virtual attribute...
def start_at
s = ScheduledSession.where("scheduled_course_id = ?", self.id).order("happening_at ASC").limit(1)
s[0].happening_at
end
... the start_at virtual attribute checks all the ScheduledSessions that belongs to the ScheduledCourse and it picks the earliest one. So start_at is the date when the first session happens.
Now I need to write in the controller so get only the records that start today and go into the future. Also I need to write another query that gets only past courses.
I can't do the following because start_at is a virtual attribute
#scheduled_courses = ScheduledCourse.where('start_at >= ?', Date.today).page(params[:page])
#scheduled_courses = ScheduledCourse.where('start_at <= ?', Date.today)
SQLite3::SQLException: no such column: start_at: SELECT "scheduled_courses".* FROM "scheduled_courses" WHERE (start_at >= '2012-03-13') LIMIT 25 OFFSET 0
You can't perform SQL queries on columns that aren't in the database. You should consider making this a real database column if you intend to do queries on it instead of a fake column; but if you want to select items from this collection, you can still do so. You just have to do it in Ruby.
ScheduledCourse.page(params).find_all {|s| s.start_at >= Date.today}
Veraticus is right; You cannot use virtual attributes in queries.
However, I think you could just do:
ScheduledCourse.joins(:scheduled_sessions).where('scheduled_courses.happening_at >= ?', Date.today)
It will join the tables together by matching ids, and then you can look at the 'happening_at' column, which is what your 'start_at' attribute really is.
Disclaimer: Untested, but should work.
I wonder if this would be solved by a subquery ( the subquery being to find the earliest date first). If so, perhaps the solution here might help point in a useful direction...