So I have a model that I want to retrieve records and group them by dates of the created_at field. But created_at is a datetime field, and I am only interested in the date part. So I am looking for something like a 2D array, first layer would a hash with date string as the key, second layer would be arrays with records. How should I do it?
{
"9/28/2012" => [record, record, record],
"9/29/2012" => [record, record, record],
"9/30/2012" => [record, record, record]
}
At top of the above, how should I do it if I want the above arrange be applied to all records retrieved from this model?
ActiveRecord group method will do what you need. In your case, the problem will be with using created_at which is a datetime, and your constraint to group by date. Since casting date-times to dates is database-specific, the code will need to be database-specific as well.
For MySql you can do:
Model.group("date(table_name.created_at)")
For SQLite you can do:
Model.group("strftime('%Y-%m-%d', table_name.created_at)")
And for PostgreSQL:
Model.group("table_name.created_at::date")
This code will unfortunately not be portable, but that may not matter to you. If it does, you can always build a wrapper method that selects the correct conversion syntax based on your DBMS.
On ActiveRecord you can do this:
2.6.4 :036 > User.group('created_at::date').count
(5.3ms) SELECT COUNT(*) AS count_all, created_at::date AS created_at_date FROM "users" GROUP BY created_at::date
=> {Sat, 27 Feb 2021=>5865
Related
Can anyone please explain me, why my db queries return empty, when I have data in my table?
Event.all
returns
...
Event Load (0.3ms) SELECT "events".* FROM "events"
=> #<ActiveRecord::Relation [#<Event id: 1, created_at: "2013-08-01 12:27:36", updated_at: "2013-08-01 12:27:36">
...
etc,
While
Event.where(created_at: Date.today)
gives me
Event Load (0.3ms) SELECT "events".* FROM "events" WHERE "events"."created_at" = '2013-08-01'
=> #<ActiveRecord::Relation []>
Where is everything?
The field created_at is a DateTime, but you are comparing it to a Date (no time).
You need to Cast the field as a Date to get all Events created today:
Event.where('CAST(events.created_at as DATE) = ?', Date.today)
Attention: The syntax may change depending on your Data-base system (PostGreSQL / MySQL, etc).
Hope this helps!
Useful link:
http://sqlserverplanet.com/tsql/cast-date
If you look at your actual query -
SELECT "events".* FROM "events" WHERE "events"."created_at" = '2013-08-01'
You are looking for a record with created_at equals 2013-08-01, but in actuality, the record you are trying to search for - the created_at field equals 2013-08-01 12:27:36.
Change your statement to do a search for created_at that contains 2013-08-01
The problem is that the created_at: attribute (assuming that it was created in the migration timestamps) also stores the time. So it will never equal a simple date. You're best option is to parse the date and then compare it.
I have a many-to-many relationship between a model User and Picture. These are linked by a join table called Picturization.
If I obtain a list of users of a single picture, i.e. picture.users -> how can I ensure that the result obtained is sorted by either creation of the Picturization row (i.e. the order at which a picture was associated to a user). How would this change if I wanted to obtain this in order of modification?
Thanks!
Edit
Maybe something like
picture.users.where(:order => "created_at")
but this created_at refers to the created_at in picturization
Have an additional column something like sequence in picturization table and define sort order as default scope in your Picturization
default_scope :order => 'sequence ASC'
If you want default sort order based on modified_at then use following default scope
default_scope :order => 'modified_at DESC'
You can specify the table name in the order method/clause:
picture.users.order("picturizations.created_at DESC")
Well, in my case, I need to sort many-to-many relation by a column named weight in the middle-table. After hours of trying, I figured out two solutions to sort many-to-many relation.
Solution1: In Rails Way
picture.users.where(:order => "created_at")
cannot return a ActiveRecord::Relation sorted by Picturization's created_at column.
I have tried to rewrite a default_scope method in Picturization, but it does not work:
def self.default_scope
return Picturization.all.order(weight: :desc)
end
Instead, first, you need to get the ids of sorted Picturization:
ids = Picturization.where(user_id:user.id).order(created_at: :desc).ids
Then, you can get the sorted objects by using MySQL field functin
picture.users.order("field(picturizations.id, #{ids.join(",")})")
which generates SQL looks like this:
SELECT `users`.*
FROM `pictures` INNER JOIN `picturizations`
ON `pictures`.`id` = `picturizations`.`picture_id`
WHERE `picturizations`.`user_id = 1#for instance
ORDER BY field(picturizations.id, 9,18,6,8,7)#for instance
Solution2: In raw SQL Way
you can get the answer directly by using an order by function:
SELECT `users`.*
FROM `pictures` INNER JOIN `picturizations`
ON `pictures`.`id` = `picturizations`.`picture_id`
WHERE `picturizations`.`user_id = 1
order by picturizations.created_at desc
In Rails 3, how do i select rows based on unique column values, i need to get all the columns for eg:
SELECT COUNT(DISTINCT date) FROM records
This only returns date column, but i want all the columns (name, date , age , created_at) columns not just the date.
Thanks for your help
The issue here is that, by definition, there may be multiple records with the same date. It requires logic in the user space to determine which of the multiple records with the unique date to use. Here's some code to get those rows:
Record.select("distinct date").each do |record|
records = Record.find_by_date record.date
puts records.count # do something with the records
end
If what you're really after is uniqueness among multiple columns, list all the relevant columns in the distinct query:
Record.select("distinct date, name, age, created_at").each do |record|
puts record.date
puts record.name
puts record.age
puts record.created_at
# ``record'' still represents multiple possible records
end
The fact that you are using distinct means that each "row" returned actually represents n rows, so the DB doesn't know which of the n rows to pull the remaining columns from. That's why it only returns the columns used in distinct. It can do no other...
I think this will help you
Model.find(:all, :select => 'DISTINCT name, date, age, created_at')
Please use it and let me know.
Model.group(:column)
For your case:
Record.group(:date)
This will return all your columns with no "date" repetitions.
For rails 3.2 and higher, Model.select('DISTINCT name, date, age, created_at')
I am trying to load the latest 10 Arts grouped by the user_id and ordered by created_at. This works fine with SqlLite and MySQL, but gives an error on my new PostgreSQL database.
Art.all(:order => "created_at desc", :limit => 10, :group => "user_id")
ActiveRecord error:
Art Load (18.4ms) SELECT "arts".* FROM "arts" GROUP BY user_id ORDER BY created_at desc LIMIT 10
ActiveRecord::StatementInvalid: PGError: ERROR: column "arts.id" must appear in the GROUP BY clause or be used in an aggregate function
LINE 1: SELECT "arts".* FROM "arts" GROUP BY user_id ORDER BY crea...
Any ideas?
The sql generated by the expression is not a valid query, you are grouping by user_id and selecting lot of other fields based on that but not telling the DB how it should aggregate the other fileds. For example, if your data looks like this:
a | b
---|---
1 | 1
1 | 2
2 | 3
Now when you ask db to group by a and also return b, it doesn't know how to aggregate values 1,2. You need to tell if it needs to select min, max, average, sum or something else. Just as I was writing the answer there have been two answers which might explain all this better.
In your use case though, I think you don't want a group by on db level. As there are only 10 arts, you can group them in your application. Don't use this method with thousands of arts though:
arts = Art.all(:order => "created_at desc", :limit => 10)
grouped_arts = arts.group_by {|art| art.user_id}
# now you have a hash with following structure in grouped_arts
# {
# user_id1 => [art1, art4],
# user_id2 => [art3],
# user_id3 => [art5],
# ....
# }
EDIT: Select latest_arts, but only one art per user
Just to give you the idea of sql(have not tested it as I don't have RDBMS installed on my system)
SELECT arts.* FROM arts
WHERE (arts.user_id, arts.created_at) IN
(SELECT user_id, MAX(created_at) FROM arts
GROUP BY user_id
ORDER BY MAX(created_at) DESC
LIMIT 10)
ORDER BY created_at DESC
LIMIT 10
This solution is based on the practical assumption, that no two arts for same user can have same highest created_at, but it may well be wrong if you are importing or programitically creating bulk of arts. If assumption doesn't hold true, the sql might get more contrieved.
EDIT: Attempt to change the query to Arel:
Art.where("(arts.user_id, arts.created_at) IN
(SELECT user_id, MAX(created_at) FROM arts
GROUP BY user_id
ORDER BY MAX(created_at) DESC
LIMIT 10)").
order("created_at DESC").
page(params[:page]).
per(params[:per])
You need to select the specific columns you need
Art.select(:user_id).group(:user_id).limit(10)
It will raise error when you try to select title in the query, for example
Art.select(:user_id, :title).group(:user_id).limit(10)
column "arts.title" must appear in the GROUP BY clause or be used in an aggregate function
That is because when you try to group by user_id, the query has no idea how to handle the title in the group, because the group contains several titles.
so the exception already mention you need to appear in group by
Art.select(:user_id, :title).group(:user_id, :title).limit(10)
or be used in an aggregate function
Art.select("user_id, array_agg(title) as titles").group(:user_id).limit(10)
Take a look at this post SQLite to Postgres (Heroku) GROUP BY
PostGres is actually following the SQL standard here whilst sqlite and mysql break from the standard.
Have at look at this question - Converting MySQL select to PostgreSQL. Postgres won't allow a column to be listed in the select statement that isn't in the group by clause.
I want to sort by two columns, one is a DateTime (updated_at), and the other is a Decimal (Price)
I would like to be able to sort first by updated_at, then, if multiple items occur on the same day, sort by Price.
In Rails 4 you can do something similar to:
Model.order(foo: :asc, bar: :desc)
foo and bar are columns in the db.
Assuming you're using MySQL,
Model.all(:order => 'DATE(updated_at), price')
Note the distinction from the other answers. The updated_at column will be a full timestamp, so if you want to sort based on the day it was updated, you need to use a function to get just the date part from the timestamp. In MySQL, that is DATE().
Thing.find(:all, :order => "updated_at desc, price asc")
will do the trick.
Update:
Thing.all.order("updated_at DESC, price ASC")
is the Rails 3 way to go. (Thanks #cpursley)
Active Record Query Interface lets you specify as many attributes as you want to order your query:
models = Model.order(:date, :hour, price: :desc)
or if you want to get more specific (thanks #zw963 ):
models = Model.order(price: :desc, date: :desc, price: :asc)
Bonus: After the first query, you can chain other queries:
models = models.where('date >= :date', date: Time.current.to_date)
Actually there are many ways to do it using Active Record. One that has not been mentioned above would be (in various formats, all valid):
Model.order(foo: :asc).order(:bar => :desc).order(:etc)
Maybe it's more verbose, but personally I find it easier to manage.
SQL gets produced in one step only:
SELECT "models".* FROM "models" ORDER BY "models"."etc" ASC, "models"."bar" DESC, "models"."foo" ASC
Thusly, for the original question:
Model.order(:updated_at).order(:price)
You need not declare data type, ActiveRecord does this smoothly, and so does your DB Engine
Model.all(:order => 'updated_at, price')
None of these worked for me!
After exactly 2 days of looking top and bottom over the internet, I found a solution!!
lets say you have many columns in the products table including: special_price and msrp. These are the two columns we are trying to sort with.
Okay, First in your Model
add this line:
named_scope :sorted_by_special_price_asc_msrp_asc, { :order => 'special_price asc,msrp asc' }
Second, in the Product Controller, add where you need to perform the search:
#search = Product.sorted_by_special_price_asc_msrp_asc.search(search_params)