I'm having an issue getting a query to work.
I'm essentially trying to write something like the following SQL, with the literal 5 replaced with a variable:
SELECT *
FROM "my_table"
WHERE 5 BETWEEN "my_table"."minimum" AND "my_table"."maximum"
This is what I have at the moment:
MyModel.where(
Arel::Nodes::Between.new(
my_variable, (MyModel.arel_table[:minimum]).and(MyModel.arel_table[:maximum])
)
)
Please ignore the way I am using arel_table, the actual query has multiple joins and is more complex, but this is the most minimum reproducible example I have to demonstrate the problem.
The error, as in the subject of the question is as follows:
undefined method `and' for #<Arel::Attributes::Attribute:0x00007f55e15514f8>
and method is for Arel::Nodes::Node i.e. MyModel.arel_attribute[:name].eq(Arel::Nodes::Quoted.new('engineersmnky')) This is an Arel::Nodes::Equality and you can chain with and.
That being said you can construct an Arel::Nodes::And for yourself via
Arel::Nodes::And.new([left,right])
Then we can pass this to the Between class like so
Arel::Nodes::Between.new(
Arel::Nodes::Quoted.new(my_variable),
Arel::Nodes::And.new([
MyModel.arel_table[:minimum],
MyModel.arel_table[:maximum]])
)
The Arel::Nodes::Quoted (also: Arel::Nodes.build_quoted(arg)) is not needed in your case since your my_variable is an Integer which can be visited and will be treated as an Arel::Nodes::SqlLiteral but I find it best to let arel decide how to handle the quoting in case your my_variable ends up being some other un-visitable Object
There are other ways to create a Between and other ways to create an And depending on what objects you are dealing with.
between is a Arel::Predication and these predications are available to Arel::Nodes::Attribute objects e.g.
MyModel.arel_table[:minimum].between([1,6])
and as mentioned is available to Arel::Nodes::Node and instances of this class provides a convenience method (create_and) for creating an And so we could do the following:
Arel::Nodes::Node.new.create_and([
MyModel.arel_table[:minimum],
MyModel.arel_table[:maximum]])
There are a number of other ways to hack this functionality together by using other Arel classes but this should get you headed in the right direction.
Related
I am new to rails. I am trying to figure out how to use model method inside the sum() sql function. I tried searching for the solution but couldn't find one. Here's the code snippet :
SUM(indents.total_payable_amount_paid) AS sum_comm_t_amount_payable_paid
I want to use method called total_payable_amount_paid defined inside indent model. But it always gives an error:
PGError: ERROR: column indents.total_payable_amount_paid does not exist.
So what's the solution for this problem? Thanks in advance!
The problem is that your SUM (which i assume it's in a query string) takes indents.total_payable_amount_paid as part of that string.
To avoid that you could use string interpolation to set the value you want, like this:
"SUM(#{indents.total_payable_amount_paid}) AS sum_comm_t_amount_payable_paid"
So, lets say indents.total_payable_amount_paid returns 250, then the above code will generate this string:
"SUM(250) AS sum_comm_t_amount_payable_paid"
But, using SUM will make no effect, since you are giving one value, so you can accomplish the same thing without it:
"#{indents.total_payable_amount_paid AS sum_comm_t_amount_payable_paid"
#=> "250 AS sum_comm_t_amount_payable_paid"
This question is about using the .try() method in rails 3 (& a bit about best practice in testing for nil also)
If I have a db object that may or may not be nil, and it has a field which is eg a delimited string, separated by commas and an optional semi-colon, and I want to see if this string contains a particular sub-string, then I might do this:
user_page = UserPage.find(id)
if user_page != nil
result = user_page.pagelist.gsub('\;', ',').split(",").include?(sub)
end
So, eg user_page.pagelist == "item_a,item_b;item_c"
I want to re-write this using .try, but cannot find a way of applying the stuff after pagelist
So I have tried passing a block (from this link http://apidock.com/rails/Object/try)
user_page.try(:pagelist) {|t| t.gsub('\;', ',').split(",").include?(sub)}
However, the block is simply ignored and the returned result is simply pagelist
I have tried other combinations of trying to apply the gsub etc part, but to no avail
How can I apply other methods to the method being 'tried'?
(I dont even know how to phrase the question in an intelligible way!)
I think you're probably coming at this wrong - the quick solution should be to add a method to your model:
def formatted_pagelist
(pagelist || "").split(/[;,]/)
end
Then you can use it where ever:
user_page.formatted_pagelist.include? sub
Strictly speaking, I'd say this goes into a decorator. If you're not using a decorator pattern in your app (look into draper!), then a model method should work fine.
In algebra if I make the statement x + y = 3, the variables I used will hold the values either 2 and 1 or 1 and 2. I know that assignment in programming is not the same thing, but I got to wondering. If I wanted to represent the value of, say, a quantumly weird particle, I would want my variable to have two values at the same time and to have it resolve into one or the other later. Or maybe I'm just dreaming?
Is it possible to say something like i = 3 or 2;?
This is one of the features planned for Perl 6 (junctions), with syntax that should look like my $a = 1|2|3;
If ever implemented, it would work intuitively, like $a==1 being true at the same time as $a==2. Also, for example, $a+1 would give you a value of 2|3|4.
This feature is actually available in Perl5 as well through Perl6::Junction and Quantum::Superpositions modules, but without the syntax sugar (through 'functions' all and any).
At least for comparison (b < any(1,2,3)) it was also available in Microsoft Cω experimental language, however it was not documented anywhere (I just tried it when I was looking at Cω and it just worked).
You can't do this with native types, but there's nothing stopping you from creating a variable object (presuming you are using an OO language) which has a range of values or even a probability density function rather than an actual value.
You will also need to define all the mathematical operators between your variables and your variables and native scalars. Same goes for the equality and assignment operators.
numpy arrays do something similar for vectors and matrices.
That's also the kind of thing you can do in Prolog. You define rules that constraint your variables and then let Prolog resolve them ...
It takes some time to get used to it, but it is wonderful for certain problems once you know how to use it ...
Damien Conways Quantum::Superpositions might do what you want,
https://metacpan.org/pod/Quantum::Superpositions
You might need your crack-pipe however.
What you're asking seems to be how to implement a Fuzzy Logic system. These have been around for some time and you can undoubtedly pick up a library for the common programming languages quite easily.
You could use a struct and handle the operations manualy. Otherwise, no a variable only has 1 value at a time.
A variable is nothing more than an address into memory. That means a variable describes exactly one place in memory (length depending on the type). So as long as we have no "quantum memory" (and we dont have it, and it doesnt look like we will have it in near future), the answer is a NO.
If you want to program and to modell this behaviour, your way would be to use a an array (with length equal to the number of max. multiple values). With this comes the increased runtime, hence the computations must be done on each of the values (e.g. x+y, must compute with 2 different values x1+y1, x2+y2, x1+y2 and x2+y1).
In Perl , you can .
If you use Scalar::Util , you can have a var take 2 values . One if it's used in string context , and another if it's used in a numerical context .
Here is the single line from one of my functions to test if any objects in my array have a given property with a matching value
Return ((From tag In DataCache.Tags Where (tag.FldTag = strtagname) Select tag).Count = 1)
WHERE....
DataCache.Tags is an array of custom objects
strtagname = "brazil"
and brazil is definitely a tag name stored within one of the custom objects in the array.
However the function continually returns false.
Can someone confirm to me that the above should or should not work.
and if it wont work can someone tell me the best way to test if any of the objects in the array contain a property with a specific value.
I suppose in summary I am looking for the equivalent of a SQL EXISTS statement.
Many thanks in hope.
Your code is currently checking whether the count is exactly one.
The equivalent of EXISTS in LINQ is Any. You want something like:
Return DataCache.Tags.Any(Function(tag) tag.FldTag = strtagname)
(Miraculously it looks like that syntax may be about right... it looks like the docs examples...)
Many Thanks for the response.
Your code did not work. Then I realised that I was comparing to an array value so it would be case sensitive.
However glad I asked the question, as I found a better way than mine.
Many thanks again !
In this question, a user commented to never use the With block in VB. Why?
"Never" is a strong word.
I think it fine as long as you don't abuse it (like nesting)
IMHO - this is better:
With MyCommand.Parameters
.Count = 1
.Item(0).ParameterName = "#baz"
.Item(0).Value = fuz
End With
Than:
MyCommand.Parameters.Count = 1
MyCommand.Parameters.Item(0).ParameterName = "#baz"
MyCommand.Parameters.Item(0).Value = fuz
There is nothing wrong about the With keyword. It's true that it may reduce readibility when nested but the solution is simply don't use nested With.
There may be namespace problems in Delphi, which doesn't enforce a leading dot but that issue simply doesn't exist in VB.NET so the people that are posting rants about Delphi are losing their time in this question.
I think the real reason many people don't like the With keyword is that is not included in C* languages and many programmers automatically think that every feature not included in his/her favourite language is bad.
It's just not helpful compared to other options.
If you really miss it you can create a one or two character alias for your object instead. The alias only takes one line to setup, rather than two for the With block (With + End With lines).
The alias also gives you a quick mouse-over reference for the type of the variable. It provides a hook for the IDE to help you jump back to the top of the block if you want (though if the block is that large you have other problems). It can be passed as an argument to functions. And you can use it to reference an index property.
So we have an alternative that gives more function with less code.
Also see this question:
Why is the with() construct not included in C#, when it is really cool in VB.NET?
The with keyword is only sideswiped in a passing reference here in an hilarious article by the wonderful Verity Stob, but it's worth it for the vitriol: See the paragraph that starts
While we are on identifier confusion. The with keyword...
Worth reading the entire article!
The With keyword also provides another benefit - the object(s) in the With statement only need to be "qualified" once, which can improve performance. Check out the information on MSDN here:
http://msdn.microsoft.com/en-us/library/wc500chb(VS.80).aspx
So by all means, use it.