How do I create an object if one is not found? This is the query I was running:
#event_object = #event_entry.event_objects.find_all_by_plantype('dog')
and I was trying this:
#event_object = EventObject.new unless #event_entry.event_objects.find_all_by_plantype('dog')
but that does not seem to work. I know I'm missing something very simple like normal :( Thanks for any help!!! :)
find_all style methods return an array of matching records. That is an empty array if no matching records are found. And an empty is truthy. Which means:
arr = []
if arr
puts 'arr is considered turthy!' # this line will execute
end
Also, the dynamic finder methods (like find_by_whatever) are officially depreacted So you shouldn't be using them.
You probably want something more like:
#event_object = #event_entry.event_objects.where(plantype: 'dog').first || EventObject.new
But you can also configure the event object better, since you obviously want it to belong to #event_entry.
#event_object = #event_entry.event_objects.where(plantype: 'dog').first
#event_object ||= #event_entry.event_objects.build(plantype: dog)
In this last example, we try to find an existing object by getting an array of matching records and asking for the first item. If there are no items, #event_object will be nil.
Then we use the ||= operator that says "assign the value on the right if this is currently set to a falsy value". And nil is falsy. So if it's nil we can build the object form the association it should belong to. And we can preset it's attributes while we are at it.
Why not use built in query methods like find_or_create_by or find_or_initialize_by
#event_object = #event_entry.event_objects.find_or_create_by(plantype:'dog')
This will find an #event_entry.event_object with plantype = 'dog' if one does not exist it will then create one instead.
find_or_initialize_by is probably more what you want as it will leave #event_object in an unsaved state with just the association and plantype set
#event_object = #event_entry.event_objects.find_or_initialize_by(plantype:'dog')
This assumes you are looking for a single event_object as it will return the first one it finds with plantype = 'dog'. If more than 1 event_object can have the plantype ='dog' within the #event_entry scope then this might not be the best solution but it seems to fit with your description.
Related
In OpsWorks Stacks, I have set a layer attribute using the custom JSON field:
{
"layer_apps" : [
"app_manager"
]
}
The app_ portion of the attribute is necessary for the workflow. At times, I need to temporarily remove the app_ portion within a cookbook. To do this, I use slice!:
node['layer_apps'].each do |app_name|
install_certs_app_name = app_name
install_certs_app_name.slice!('app_') # 'app_manager' => 'manager'
# snip
end
However, once this is done, even though app_name isn't being directly modified, each node['layer_apps'] attribute gets sliced, which carries on to subsequent cookbooks and causes failures. The behaviour I expected was that slice! would modify app_name, and not the current node['layer_apps'] attribute. Thinking that app_name was a link to the attribute rather than being it's own variable, I tried assigning its value to a separate variable (install_certs_app_name and similar in other cookbooks), but the behaviour persisted.
Is this expected behaviour in Ruby/Chef? Is there a better way to be excluding the app_ prefix from the attribute?
app_name is being directly modified. That's the reason for the bang ! after the method... so that you're aware that the method mutates the object.
and app_name and install_certs_app_name are referencing the same object.
Note that slice and slice! both return "app_" but the bang object mutates the caller by removing the sliced text.
If you did
result = install_certs_app_name.slice!('app_')
puts result
==> app_
puts install_certs_app_name
--> manager
Try (instead)
install_certs_app_name = app_name.dup
install_certs_app_name.slice!('app_')
So you have two separate objects.
Alternatively,
install_certs_app_name = app_name.sub('app_', '')
In case you'd want a variable sliced, what you'll is the non-destructive version:
str.slice and not str.slice!
These are often referred to as Bang-methods, and replace the variable in place.
Below is an example with the .downcase method. This is the same principle for .slice.
EDIT:
However, since .slice returns the part that's been cut out, you could just remove the app_-part .sub like
"app_manager".sub("app_",'') #=> "manager"
http://ruby-for-beginners.rubymonstas.org/objects/bangs.html
https://ruby-doc.org/core-2.2.0/String.html#method-i-slice
When you assigning app_name to install_certs_app_name you still referencing to the same object. In order to create new object you can do:
install_certs_app_name = app_name.dup
New object with the same value is created. And slicing install_certs_app_name does not affect app_name this way.
I have this pretty simple method:
def update_context(msg, session, sender)
previous_context = session.context
session.update(context: intent_determination(msg, session.context, sender))
session.update(context: brand_determination(msg, session.context))
session.update(context: style_determination(msg, session.context))
session.update(context: price_range_determination(msg, session.context))
session.update(context: size_determination(msg, session.context))
p previous_context
p session.context
p (previous_context == session.context)
unless session.context.size == 0
if previous_context == session.context
session.context["intent"] = "lost"
session.save
end
end
end
My problem is certainly due to a stupid mistake I can't see but please bear with me on this one, I really can't see it.
As you can see, I "save" the session's context in a previous_context variable at the beginning of the method. Then, I'm running a few updates on the context.
However, when I print previous_context, session.context and previous_context == session.context, I get the same result for the first two, and true for the last one.
How is this possible ? I assigned to previous_context the value of session.context before updating it. And then, previous_context has the same value as session.context after I've updated it.
I really can't see where I screwed up here, or there is definitely something I don't understand.
previous_context = session.context makes the previous_context variable point to the same object as session.context. If you want to change one without affecting the other, you'll need to create a copy of session.context to store in previous_context.
in Ruby, variables are just references to objects, so what you are doing there is merely creating a new reference to the same object. If you wish to save the previous state you will have to copy the entire object.
See this answer for a more graphic explanation.
I am new to Ruby, so let me describe the context of my problem first:
I have a json as input which has the following key / value pair:
{
"service": "update"
}
The value has many different values for example: insert,delete etc.
Next there is a method x which handles the different requests:
def x(input)
case input[:service]
services = GenericService.new
when "update"
result = services.service(UpdateService.new,input)
when "insert"
result = services.service(InsertService.new,input)
when "delete"
result = services.service(DeleteService.new,input)
....
....
else
raise "Unknown service"
end
puts JSON.pretty_generate(result)
end
What is bothering me is that I still need to use a switch statement to check the String values (reminds me of 'instance of' ugh..). Is there a cleaner way (not need to use a switch)?
Finally I tried to search for an answer to my question and did not succeed, if however I missed it feel free to comment the related question.
Update: I was thinking to maybe cast the string to the related class name as follows: How do I create a class instance from a string name in ruby? and then call result = services.services(x.constantize.new,input) , then the class names ofcourse needs to match the input of the json.
You can try something like:
def x(input)
service_class_name = "#{input[:service].capitalize}Service"
service_class = Kernel.const_get(service_class_name)
service_class.new(input).process
end
In addition you might want to check if this is a valid Service class name at all.
I don't understand why you want to pass the service to GenericService this seems strange. let the service do it's job.
If you're trying to instatiate a class by it's name you're actually speaking about Reflection rather than Polymorphism.
In Ruby you can achieve this in this way:
byName = Object.const_get('YourClassName')
or if you are in a Rails app
byName= 'YourClassName'.constantize
Hope this helps
Just first thoughts, but you can do:
eval(services.service("#{input[:service].capitalize}Service.new, #{input})") if valid_service? input[:service]
def valid_service?
w%(delete update insert).include? input[:service]
end
As folks will no doubt shout, eval needs to be used with alot of care
I have an array of strings, that represent existing object names.
JoesDev = Dev.new
MarksDev = Dev.new
SamsDev = Dev.new
devices=['JoesDev', 'MarksDev', 'SamsDev' ]
i'd like to iterate over the devices array, while calling a method on the objects that each item in the array is named after.
i.e;
JoesDev.method_name
MarksDev.method_name
SamsDev.method_name
how can i do this? thx.
devices.each{|name| self.class.const_get(name).method_name}
You can use the const_get method from Module to have Ruby return the constant with the given name. In your case, it will return the Dev instance for whatever device name you give it.
Using .each to iterate the items, your code could look like
devices.each do |device_name|
device = self.class.const_get(device_name)
device.method_name
end
# Which can be shortened to
devices.each{ |dev| self.class.const_get(dev).method_name }
However, there are better ways to implement this type of thing. The most common way is using a Hash. In your example, the list of devices could look something like
devices = {
joe: Dev.new,
mark: Dev.new,
sam: Dev.new
}
Then, iterating over the devices is as simple as
devices.each do |dev|
dev.method_name
end
# Or
devices.each{ |dev| dev.method_name }
Extra: If you want to get a little fancy, you can use the block version of Hash::new to make adding new devices extremely simple.
# Create the hash
devices = Hash.new{ |hash, key| hash[key] = Dev.new }
# Add the devices
devices['joe']
devices['mark']
devices['sam']
This kind of hash works exactly the same as the one shown above, but will create a new entry if the given key cannot be found in the hash. A potential problem with this design, then, is that you can accidentally add new devices if you make a typo. For example
devices['jon'] # This would make a new Dev instance, which may be undesirable.
Well one way is surely to use eval, a method that allows you to execute arbitrary strings as if they were code.
So, in your example:
var_names.each{ |var_name| eval("#{var_name}.some_method") }
Needless to say, it is very dangerous to let unfiltered strings to be used as code, very bad things™ may happen!
I have a collection of objects. There are 3 properties in each object
'id', 'name', 'is_primary'
The collection of objects will usually have anywhere from 1 to 5 objects.
What I want to do is check the collection to see if is_primary is true. If so output the name, or at least return it.
I want to do this in 1 line of code if possible. I am trying to slim up this one line for erb output in rails. Later in the page i'll output them all. I thought I had it, but if I return nil it adds extra space which shifts all the html oddly.
Thanks.
Hmm, this doesn't quite work if no element is_primary...I'm still thinking...
c.detect(&:is_primary).name
Ok, how about:
((a = c.detect(&:is_primary)) && a.name).to_s
As it happens, it is OK in an erb template for the <%= expression to return nil, that just results in an empty string, so for that case you can use:
(a = c.detect(&:is_primary)) && a.name
Update: Responding to the first comment, I do have a test case that I didn't post...
class A; attr_accessor :is_primary, :name, :id; end
t = [A.new, A.new, A.new, (a = A.new; a.name = 'xyz'; a.is_primary = true; a)]
puts (a = t.detect(&:is_primary)) && a.name
puts ((a = [].detect(&:is_primary)) && a.name).to_s
Complementing #DigitalRoss, you can also write:
collection.detect(&:is_primary).try(:name) || "default_if_no_element_or_name"
(well, to be honest I prefer Ick's maybe over Rails' try: c.detect(&:is_primary).maybe.name)
Side note: IMHO a flag that can only be active for a row it's not such a good idea. You may have inconsistent states with more than one being active and you'll have worry about it when updating (transactions, and so on). Try to store the PK reference somewhere else (a parent model? a state model?).
I want to do this in 1 line of code if possible. I am trying to slim up this one line for erb output in rails. Later in the page i'll output them all.
No need for one-liners (funny since I just wrote one): move the code to yous models or helpers as appropriate and keep your views pristine.