Is it possible to retrieve a count of distinct records based on a field value if the field needs to be interrogated (ideally, using ActiveRecord alone)?
For example, the following returns a count of unique records based on the 'created_at' field:
Record.count('created_at', :distinct => true)
However, is it possible to get a count of, say, unique days based on the 'created_at' field in a similar way?
A naive ActiveRecord example to explain my intent being:
Record.count('created_at'.day, :distinct => true)
(I know the string 'created_at' isn't a 'Time', but that's the sort of query I'd like to ask ActiveRecord.)
You need to group the records. For example
Record.group('DATE(created_at)').count('created_at')
tells you the number of rows created on each particular date, or
Record.group('DAYOFWEEK(created_at)').count('created_at')
would tell you the number of rows created on individual days of the week.
Beware that this with the usual active record setup this will do date calculations in UTC, if you want to do your calculations in a specific timezone you'll have to add that conversion to the group statement.
Related
I have several records in my database, the table has a column named "weekday" where I store a weekday like "mon" or "fri". Now from the frontend when a user does search the parameters posted to the server are startday and endDay.
Now I would like to retrieve all records between startDay and endDay. We can assume startDay is "mon" and endDay is "sun". I do not currently know how to do this.
Create another table with the names of the days and their corresponding number. Then you'd just need to join up your current table with the days table by name, and then use the numbers in that table to do your queries.
Not exactly practical, but it is possible to convert sun,mon,tue to numbers using MySQL.
Setup a static year and week number like 201610 for the 10th week of this year, then use a combination of DATE_FORMAT with STR_TO_DATE:
DATE_FORMAT(STR_TO_DATE('201610 mon', '%X%V %a'), '%w')
DATE_FORMAT(STR_TO_DATE('201610 sun', '%X%V %a'), '%w')
DATE_FORMAT(STR_TO_DATE('201610 tue', '%X%V %a'), '%w')
These 3 statements will evaluate to 0,1,2 respectively.
The main thing this is doing is converting the %a format (Sun-Sat) to the %w format (0-6)
well i don't know the architecture of your application as i think storing and querying a week day string is not appropriate, but i can tell you a work around this.
make a helper function which return you an array of weekdays in the range i-e
function getWeekDaysArray($startWeekDay, $endWeekDay) {
returns $daysArray['mon','tue','wed'];
}
$daysRangeArray = getWeekDaysArray('mon', 'wed');
now with this array you can query in table
DB::table('TableName')->whereIn('week_day', $daysRangeArray)->get();
Hope this help
This is too tough for me guys. It's for Jeremy!
I have two tables (although I can also envision needing to join a third table) and I want to sum one field and count rows, in the same, table while joining with another table and return the result in json format.
First of all, the data type field that needs to be summed, is numeric(10,2) and the data is inserted as params['amount'].to_f.
The tables are expense_projects which has the name of the project and the company id and expense_items which has the company_id, item and amount (to mention just the critical columns) - the "company_id" columns are disambiguated.
So, the following code:
expense_items = DB[:expense_projects].left_join(:expense_items, :expense_project_id => :project_id).where(:project_company_id => company_id).to_a.to_json
works fine but when I add
expense_total = expense_items.sum(:amount).to_f.to_json
I get an error message which says
TypeError - no implicit conversion of Symbol into Integer:
so, the first question is why and how can this be fixed?
Then I want to join the two tables and get all the project names form the left (first table) and sum amount and count items in the second table. I have tried
DB[:expense_projects].left_join(:expense_items, :expense_items_company_id => expense_projects_company_id).count(:item).sum(:amount).to_json
and variations of this, all of which fails.
I would like a result which gets all the project names (even if there are no expense entries and returns something like:
project item_count item_amount
pr 1 7 34.87
pr 2 0 0
and so on. How can this be achieved with one query returning the result in json format?
Many thanks, guys.
Figured it out, I hope this helps somebody else:
DB[:expense_projects___p].where(:project_company_id=>user_company_id).
left_join(:expense_items___i, :expense_project_id=>:project_id).
select_group(:p__project_name).
select_more{count(:i__item_id)}.
select_more{sum(:i__amount)}.to_a.to_json
I am using the Ruby Mongoid gem and trying to create a query to retrieve the last 100 documents from a collection. Rather than using Mongoid, I would like to create the query using the underlying driver (Moped). The Moped documentation only mentions how to retrieve the first 100 records:
session[:my_collection].find.limit(100)
How can I retrieve the last 100?
I have found a solution, but you will need to sort collection in descending order. If you have a field id or date you would do:
Method .sort({fieldName: 1 or -1})
The 1 will sort ascending (oldest to newest), -1 will sort descending (newest to oldest). This will reverse entries of your collection.
session[:my_collection].find().sort({id:-1}) or
session[:my_collection].find().sort({date:-1})
If your collection contain field id (_id) that identifier have a date embedded, so you can use
session[:my_collection].find().sort({_id:-1})
In accordance with your example using .limit() the complete query will be:
session[:my_collection].find().sort({id:-1}).limit(100);
Technically that query isn't finding the first 100, that's essentially finding 100 random documents because you haven't specified an order. If you want the first then you'd have to say explicitly sort them:
session[:my_collection].find.sort(:some_field => 1).limit(100)
and to reverse the order to find the last 100 with respect to :some_field:
session[:my_collection].find.sort(:some_field => -1).limit(100)
# -----------------------------------------------^^
Of course you have decide what :some_field is going to be so the "first" and "last" make sense for you.
If you want them sorted by :some_field but want to peel off the last 100 then you could reverse them in Ruby:
session[:my_collection].find
.sort(:some_field => -1)
.limit(100)
.reverse
or you could use use count to find out how many there are then skip to offset into the results:
total = session[:my_collection].find.count
session[:my_collection].find
.sort(:some_field => 1)
.skip(total - 100)
You'd have to check that total >= 100 and adjust the skip argument if it wasn't of course. I suspect that the first solution would be faster but you should benchmark it with your data to see what reality says.
I'm working on a program that keeps a list of physical advertisement spots and their reservations (a date range). The program needs to be able to find "open spots" for ads within an ad category.
I have three tables: AdTypes, AdPlaces and Reservations. Currently, I'm implementing a query that searches for reservations where the dates don't collide with the date range the user selected, and returns the AdType items as a list. This method works if every AdType has had an reservation at some point, but it doesn't list AdTypes that are not found in the Reservations table.
The filtering is done in PreProcessQuery of an AdTypes query, as such:
query = query.Where(r => r.Reservations.Any(res => (res.Begindate > Begindate && Enddate < res.Enddate) || (res.Enddate < Begindate && Enddate > res.Begindate)));
How can I "extend" the query so that all those AdTypes that have no reservations would be listed alongside "expired" AdType reservations?
Maybe I'm mssing something, or not quite understanding what you want, but how can you expect see AdTypes that don't exist?
Is it that you want to see the AdTypes that don't have any reservations during the period you're testing for?
If so, I imagine you'd have to use the adType entity as the basis of your screen, not the Reservation entity (with a query based on AdType, not Reservation). That way you can produce a list of AdTypes that don't have any overlapping reservations.
Does that make any sense?
I run the following in Rails:
coe = trackers.where(:trackable_type => "Mailing").count(:group => 'DATE(created_at)')
which returns a hash with the items nicely grouped.
However I want to group not by date only by date but by the hour, but not the minutes or seconds.
How can I run this query so all the heavy lifting is done by Postgresql and not convert after I get the data set back?
My mistake I want to group by Date and Hour. Sorry for the confusion.
It sounds like you want the date_trunc() function:
http://www.postgresql.org/docs/current/interactive/functions-datetime.html#FUNCTIONS-DATETIME-TABLE
I want to group not by date but by the hour, but not the minutes or
seconds.
Bold emphasis mine.
So date_trunc()is probably not what you want, but EXTRACT or date_part():
coe = trackers.where(:trackable_type => "Mailing").count(:group => 'EXTRACT (hour FROM created_at)')