I have the following Structure for events and there recurrences
Event: (id, name, venue)
has_many :occurences
Occurence(id, date, event_id)
belongs_to :event
I want to get events (only event data) that have a recurrence greater than today
(occurences.date>Date.Today)
Events should be ordered by the date of their next recurrence(greater than today) in chronological order
The following query gives me event data alright but it doesn't let me order
Event.joins(:occurences).where("occurences.date>?",DateTime.now).distinct#.order('occurences.date')
but I can't order it since it says
Expression #1 of ORDER BY clause is not in SELECT list, references
column 'eventdatabase.occurences.starts'
I need to use distinct to ensure I get only one event regardless of how many occurences it has
I am using mysql and rails5
I'm interpreting
Events should be ordered by the date of their next recurrence(greater than today) in chronological order
as earliest occurance.date.
Event.joins(:occurences)
.where("occurences.date>?",DateTime.now)
.group(:id)
.select('events.*, min(occurences.date) as next_recurrence')
.order('next_recurrence')
A group by is like a more powerful distinct. You will get one row (for one "group"). Technically we should group by events.*, but mysql will cheat and let us group by a primary key to do the same thing.
When doing a group by, the aggregate functions, such as min work on the group.
For this task the SQL query may look like:
SELECT E.id AS event_id, MIN(OC.date) AS next_date
FROM events E
JOIN occurences OC ON OC.event_id = E.id
WHERE OC.date > NOW()
GROUP BY E.id
ORDER BY MIN(OC.date);
Here is sandbox: http://rextester.com/HCTE57488
So I guess the Ruby code will be:
Event.joins(:occurences)
.where('occurences.date > ?', DateTime.now)
.group('events.id')
.order('min(occurences.date)')
Related
I am trying to query my PostgreSQL database to get the latest (by created_at) and distinct (by user_id) Activity objects, where each user has multiple activities in the database. The activity object is structured as such:
Activity(id, user_id, created_at, ...)
I first tried to get the below query to work:
Activity.order('created_at DESC').select('DISTINCT ON (activities.user_id) activities.*')
however, kept getting the below error:
ActiveRecord::StatementInvalid: PG::InvalidColumnReference: ERROR: SELECT DISTINCT ON expressions must match initial ORDER BY expressions
According to this post: PG::Error: SELECT DISTINCT, ORDER BY expressions must appear in select list, it looks like The ORDER BY clause can only be applied after the DISTINCT has been applied. This does not help me, as I want to get the distinct activities by user_id, but also want the activities to be the most recently created activities. Thus, I need the activities to be sorted before getting the distinct activities.
I have come up with a solution that works, but first grouping the activities by user id, and then ordering the activities within the groups by created_at. However, this takes two queries to do.
I was wondering if what I want is possible in just one query?
This should work, try the following
Solution 1
Activity.select('DISTINCT ON (activities.user_id) activities.*').order('created_at DESC')
Solution 2
If not work Solution 1 then this is helpful if you create a scope for this
activity model
scope :latest, -> {
select("distinct on(user_id) activities.user_id,
activities.*").
order("user_id, created_at desc")
}
Now you can call this anywhere like below
Activity.latest
Hope it helps
I have an index of active job positions. Currently, they're sorted by the most recent i.e. created_at. However, recently i've added in a renewal feature that updates a renewal_date attribute without updating the created_at.
What I want to achieve is to sort the list in descending order using both renewal_date and created_at.
jobs = Job.where(active: true).reorder("renewal_date DESC NULLS LAST", "created_at DESC")
With this code, the renewed job will always be at the top regardless of how many new jobs are created. How do I sort it so it checks for the date for both attributes and sorts it according to most recent?
Your code will order first by renewal_date with nulls at the end, and then will look at the created_at if two records have the same renewal_date.
I assume that what you want to do is something like "max(renewal_date, created_at)", which will take the "last modification date", or another custom way to compare the two fields.
If then, you can find your answer here : merge and order two columns in the same model
Job.where(active: true).reorder('GREATEST(renewal_date, created_at) DESC')
Let try a standard SQL, so it can work with all types of database:
Job.where(active: true).order('CASE WHEN renewal_date IS NULL THEN created_at ELSE renewal_date END DESC')
I have the following code to join two tables microposts and activities with micropost_id column and then order based on created_at of activities table with distinct micropost id.
Micropost.joins("INNER JOIN activities ON
(activities.micropost_id = microposts.id)").
where('activities.user_id= ?',id).order('activities.created_at DESC').
select("DISTINCT (microposts.id), *")
which should return whole micropost columns.This is not working in my developement enviornment.
(PG::InvalidColumnReference: ERROR: for SELECT DISTINCT, ORDER BY expressions must appear in select list
If I add activities.created_at in SELECT DISTINCT, I will get repeated micropost ids because the have distinct activities.created_at column. I have done a lot of search to reach here. But the problem always persist because of this postgres condition to avoid random selection.
I want to select based on order of activities.created_at with distinct micropost _id.
Please help..
To start with, we need to quickly cover what SELECT DISTINCT is actually doing. It looks like just a nice keyword to make sure you only get back distinct values, which shouldn't change anything, right? Except as you're finding out, behind the scenes, SELECT DISTINCT is actually acting more like a GROUP BY. If you want to select distinct values of something, you can only order that result set by the same values you're selecting -- otherwise, Postgres doesn't know what to do.
To explain where the ambiguity comes from, consider this simple set of data for your activities:
CREATE TABLE activities (
id INTEGER PRIMARY KEY,
created_at TIMESTAMP WITH TIME ZONE,
micropost_id INTEGER REFERENCES microposts(id)
);
INSERT INTO activities (id, created_at, micropost_id)
VALUES (1, current_timestamp, 1),
(2, current_timestamp - interval '3 hours', 1),
(3, current_timestamp - interval '2 hours', 2)
You stated in your question that you want "distinct micropost_id" "based on order of activities.created_at". It's easy to order these activities by descending created_at (1, 3, 2), but both 1 and 2 have the same micropost_id of 1. So if you want the query to return just micropost IDs, should it return 1, 2 or 2, 1?
If you can answer the above question, you need to take your logic for doing so and move it into your query. Let's say that, and I think this is pretty likely, you want this to be a list of microposts which were most recently acted on. In that case, you want to sort the microposts in descending order of their most recent activity. Postgres can do that for you, in a number of ways, but the easiest way in my mind is this:
SELECT micropost_id
FROM activities
JOIN microposts ON activities.micropost_id = microposts.id
GROUP BY micropost_id
ORDER BY MAX(activities.created_at) DESC
Note that I've dropped the SELECT DISTINCT bit in favor of using GROUP BY, since Postgres handles them much better. The MAX(activities.created_at) bit tells Postgres to, for each group of activities with the same micropost_id, sort by only the most recent.
You can translate the above to Rails like so:
Micropost.select('microposts.*')
.joins("JOIN activities ON activities.micropost_id = microposts.id")
.where('activities.user_id' => id)
.group('microposts.id')
.order('MAX(activities.created_at) DESC')
Hope this helps! You can play around with this sqlFiddle if you want to understand more about how the query works.
Try the below code
Micropost.select('microposts.*, activities.created_at')
.joins("INNER JOIN activities ON (activities.micropost_id = microposts.id)")
.where('activities.user_id= ?',id)
.order('activities.created_at DESC')
.uniq
I have two tables,
Order (ID, Value)
and
OrderType (ID, Name [Quote, Sale, Purchase, etc])
I want to get the total number of orders in each type (count) and the total value of those orders per type (sum)
I can get these individually using
Order.group(:order_type).count(:id)
and
Order.group(:order_type).sum(:value)
I would like to perform these in one query, the equivalent to the following SQL
SELECT
order_types.id, Count(*) as total_count, Sum(orders.value) As total_value
FROM
order
JOIN
order_types ON orders.order_type_id = order_types.ID
GROUP BY
order_types.id
The query should also return the full OrderType object so I can display the name in my view
Since ActiveRecord does not support multiple aggregation functions in the same query, you need to do a bit of raw SQL to achieve this.
grouped_sales = OrderType
.select('order_types.id, order_types.name,
sum(orders.value) as sale, count(*) as purchase')
.join('JOIN orders ON orders.order_type_id = order_types.id')
.group('order_types.id')
The point to note here is that you need to use an existing column in OrderType as the alias for your aggregated columns. Here you will get the OrderType object as well.
To access the result:
id -> grouped_sales.first.id
name -> grouped_sales.first.name
total orders -> grouped_sales.first.sale
order value -> grouped_sales.first.purchase
There is even better solution, just:
.pluck('sum(orders.value), count(*)').first
Nowadays pluck+arel will do the job.
model = Model.arel_table
Model.group(:order_type).pluck(model[:id].count, model[:value].sum)
Also appending with .order(:order_type) may be needed if there applied default ordering by ID.
How do I retrieve a set of records, ordered by count in Arel? I have a model which tracks how many views a product get. I want to find the X most frequently viewed products over the last Y days.
This problem has cropped up while migrating to PostgreSQL from MySQL, due to MySQL being a bit forgiving in what it will accept. This code, from the View model, works with MySQL, but not PostgreSQL due to non-aggregated columns being included in the output.
scope :popular, lambda { |time_ago, freq|
where("created_on > ?", time_ago).group('product_id').
order('count(*) desc').limit(freq).includes(:product)
}
Here's what I've got so far:
View.select("id, count(id) as freq").where('created_on > ?', 5.days.ago).
order('freq').group('id').limit(5)
However, this returns the single ID of the model, not the actual model.
Update
I went with:
select("product_id, count(id) as freq").
where('created_on > ?', time_ago).
order('freq desc').
group('product_id').
limit(freq)
On reflection, it's not really logical to expect a complete model when the results are made up of GROUP BY and aggregate functions results, as returned data will (most likely) match no actual model (row).
you have to extend your select clause with all column you wish to retrieve. or
select("views.*, count(id) as freq")
SQL would be:
SELECT product_id, product, count(*) as freq
WHERE created_on > '$5_days_ago'::timestamp
GROUP BY product_id, product
ORDER BY count(*) DESC, product
LIMIT 5;
Extrapolating from your example, it should be:
View.select("product_id, product, count(*) as freq").where('created_on > ?', 5.days.ago).
order("count(*) DESC" ).group('product_id, product').limit(5)
Disclaimer: Ruby syntax is a foreign language to me.