I have an array with products, I need to display only 1 product, with the largest number in available_amount. How can i do this?
How do I iterate to display products with parameters:
- #part.wh_ps.sort_by(&:available_amount).each do |whp|
product number1: available_amount: 2;
product number2: available_amount: 5;
If those objects are mapped from the database being the result of a query and you already have them in memory, then you could use Enumerable#max_by:
#part.wh_ps.max_by(&:available_amount)
It should return the object within that array with the biggest available_amount if any.
If you need the one with the lowest available_amount, then Enumerable#min_by, and if you need both Enumerable#minmax_by.
However if that's not the case and you're hitting the database again, you could consider making the exact query using SQL (ActiveRecord) asking for the row with the biggest value for the given column.
Related
I am hardly trying to find one comparison of result.blank? and result[0] so finally today when i was checking one query with these two methods.
Here the code, result variable is #categories, which is an ActiveRecord result
This blank check calling one extra db call like SELECT COUNT(*) AS count_all
if #categories.blank?
end
But here that extra query is not showing there.
if #categories[0]
end
Is there any logic behind that? I couldn't find that
It is important to note that assigning a ActiveRecord query to a variable does not return the result of the query. Something like this:
#categories = Category.where(public: true)
Does not return an array with all categories that are public. Instead it returns an Relation which defines an query. The query to the database is execute once you call a method in the relation that needs to return the actual record, for example each, load, count.
That said: When you call blank? on a relation Rails needs to know it the relation will not return an empty array. Therefore Rails executes an query like:
SELECT COUNT(*) FROM categories WHERE public = 1
Because that queries is much faster that fetching all records when the only thing you need to know if there are any matching records.
Whereas #categories[0] works differently. Here it need to load all records to have an array holding all macthing categories and than return the first record in that array.
At this point both version ran only on query to the database. But I guess your next step would be to iterate over the records if there were any. If you used the first version (blank?) then the objects were not loaded, they were only counted. Therefore Rails would need to query for the actual records, what would result in a second query. The second exmaple ([0]) has the records already loaded, therefore not seconds query in needed.
I have a Parse class called Book. Inside it is a column pageNumber.
I want to query the most recent objects for each page number. There can be more than one object with the same pageNumber which is why I only want to query the most recent object for each value under pageNumber.
So, if there are six different objects with a value of (4) under pageNumber in the class Book how would I get just the most recently created object. Not just for pageNumber (4) but for all unique pageNumbers 1-50.
For a specific page number :
You should order the result by createdAt date, and set the limit of the query to 1 :
query.descending("createdAt");
query.limit(1);
For each page number, you can either iterate on every possible page number value, or retrieve everything and iterate other the results, selecting the most recent object for the same page number (using a hash table)
Hopefully this is a little clearer. I'm sorry but I'm very new to coding in general. I have multiple tables that I have to query in succession in order to get to the correct array that I need. The following logic for the query is as follows:
this gives me an array based upon the store :id
store = Stores.find(params[:id])
this gives me another array based upon the param .location found in the table store where that value equals the row ID in the table Departments
department = Departments.find(store.location)
I need to preform one last query but in order to do so I need to figure out which day of the meeting is needed. In order to do this I have to create the parameter day_of_meeting found in the table Stores. I try to call it from the array above and create a new variable. In the Table Departments, I there are params such as day_1, day_2 and so on. I need to be able to call something like department.day_1 or department.day_2. Thus, I'm trying to actually create the variable by join the words "department.day_" to the variable store.day_of_meeting which would equal some integer, creating department.day_1...
which_day = ["department.day_", store.day_of_meeting].join("")
This query finds uses the value found from the variable department.day_1 to query table Meeting to find the values in the corresponding row.
meeting = Meeting.find(which_day)
Does this make my problem any clearer to understand?
findmethod can only accept parameters like Meeting.find(1) or Meeting.find("1-xx").
so, what you need is Meeting.find(department.send("day_" + store.day_of_meeting.to_s))
Hope to help!
So far I have a query with a result set (in a temp table) with several columns but I am only concerned with four. One is a customer ID(varchar), one is Date (smalldatetime), one is Amount(money) and the last is Type(char). I have multiple rows with the same custmer ID and want to evaluate them based on Date, Amount and Type. For example:
Customer ID Date Amount Type
A 1-1-10 200 blue
A 1-1-10 400 green
A 1-2-10 400 green
B 1-11-10 100 blue
B 1-11-10 100 red
For all occurrences of A I want to compare them to identify only one, first by earliest date, then by greatest Amount, then if still tied by comparing Types. I would then return one row for each customer.
I would provide some of the query but I am at home now after spending two days trying to get a correct result. It looks something like this:
(query to populate #tempTable)
GROUP BY customer_id
HAVING date_cd =
(SELECT MIN(date_cd)
FROM order_table ot
WHERE ot.customerID = #tempTable.customerID
)
OR date_cd IS NULL
I assume the HAVING would result in only one row per customer_id. This did not end up being the case since there were some ties there.
I am not sure I can do the OR - there are some with NULL values here - and it did not account for the step to the next comparison if they were all the same anyway. I am not seeing a way to avoid doing some row processing of the temp table with some kind of IF or WHERE loop.
As I write I am thinking maybe I use #tempTable.date_cd in the HAVING clause instead of looking at the original table. but that should return the same dates?
Am I on the right track or is there something missing? Suggestions? More info??
try below query :-
select * from #tempTable
GROUP BY customer_id
HAVING isnull(date_cd,"1900/01/01") =min(isnull(date_cd,"1900/01/01"))
Hi is it possible using Entity Framework and/or linq to select a certain number of rows? For example i want to select rows 0 - 500000 and assign these records to the List VariableAList object, then select rows 500001 - 1000000 and assign this to the List VariableBList object, etc. etc.
Where the Numbers object is like ID,Number,DateCreated, DateAssigned, etc.
Sounds like you're looking for the .Take(int) and .Skip(int) methods
using (YourEntities db = new YourEntities())
{
var VariableAList = db.Numbers
.Take(500000);
var VariableBList = db.Numbers
.Skip(500000)
.Take(500000);
}
You may want to be wary of the size of these lists in memory.
Note: You also may need an .OrderBy clause prior to using .Skip or .Take--I vaguely remember running into this problem in the past.