# What this line means exactly? (@@active\_connections ||= {})

**URL:** <https://www.ruby-forum.com/t/what-this-line-means-exactly-active-connections/96828>\
**Category:** Rails\
**Created:** [January 24, 2006, 9:35pm UTC](https://www.ruby-forum.com/t/what-this-line-means-exactly-active-connections/96828 "2006-01-24T21:35:00Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![sayoyo](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/ba8739/32.png) [@sayoyo](https://www.ruby-forum.com/u/sayoyo)\
**Post date:** [January 24, 2006, 9:35pm UTC](https://www.ruby-forum.com/t/what-this-line-means-exactly-active-connections/96828/1 "2006-01-24T21:35:00Z")

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Hi,

I have problem to understand this line does exactly?  
this is from the class ConnectionSpecficication,  
methode “active\_connections”

def self.active\_connections #:nodoc:  
if allow\_concurrency  
Thread.current[‘active\_connections’] ||= {}  
else  
@@active\_connections ||= {}  
end  
end

what the line “@@active\_connections ||= {}” means  
exactly?  
as I undertands is if the class varaible  
@@active\_connections has a value then return the  
value, otherwise return {}, but what is {}, a bloc of  
nothing??? or what?

Thanks you very much

Saiho

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<div class="post-metadata">

**Author:** ![sayoyo](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/ba8739/32.png) [@sayoyo](https://www.ruby-forum.com/u/sayoyo)\
**Post date:** [January 24, 2006, 9:41pm UTC](https://www.ruby-forum.com/t/what-this-line-means-exactly-active-connections/96828/2 "2006-01-24T21:41:05Z")

</div>

On 1/24/06, Saiho Y. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> ```
> @@active_connections ||= {}
> 
> ```
> 
> end  
> end
> 
> what the line “@@active\_connections ||= {}” means  
> exactly?  
> as I undertands is if the class varaible  
> @@active\_connections has a value then return the  
> value, otherwise return {}, but what is {}, a bloc of  
> nothing??? or what?

a = 2  
a ||= 3

# a is 2

b ||= 3

# b is 3

So, ||= will set a variable to some value if the variable doesn’t have  
a value yet.

{} is a hash. Could’ve used:  
@@active\_connections ||= Hash.new  
instead.

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<div class="post-metadata">

**Author:** ![sayoyo](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/ba8739/32.png) [@sayoyo](https://www.ruby-forum.com/u/sayoyo)\
**Post date:** [January 24, 2006, 9:41pm UTC](https://www.ruby-forum.com/t/what-this-line-means-exactly-active-connections/96828/3 "2006-01-24T21:41:05Z")

</div>

{} is an anonymous hash

It seems to be saying

“return whatever it is, or initialize it as an empty hash and return  
it.”

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<div class="post-metadata">

**Author:** ![sayoyo](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/ba8739/32.png) [@sayoyo](https://www.ruby-forum.com/u/sayoyo)\
**Post date:** [January 25, 2006, 1:14am UTC](https://www.ruby-forum.com/t/what-this-line-means-exactly-active-connections/96828/4 "2006-01-25T01:14:02Z")

</div>

Yeah, it will use @@active\_connections if it has been set, otherwise it  
will set @@active\_connections to an empty hash and return it.

One thing about this feature has caught me out a couple of times. Look  
at this:

x = false  
x ||= 4

# x = 4 !

x is set to 4 even though it has already been set to false. It does this  
because ||= is simply a shortcut for x = x || 4, and (false || 4)  
results in 4.

Interesting 🙂

-Jonny.

Kelly Dwight F. wrote:

> {} is an anonymous hash
> 
> It seems to be saying
> 
> “return whatever it is, or initialize it as an empty hash and return  
> it.”