Chef node attribute definition: define a field by reference other field

Question

How can I define a ruby Hash by this way?

default['mgmt']['query'] = {
    'default_interval' => {'diff' => 3600, 'snapshot' => 86400 * 7},
    'tables' => {
        'deb_packages' => default_interval,
        'rpm_packages' => default_interval,
        ...
    }
}

EDIT: above code is for Chef recipe's attributes/default.rb

I am looking for a way to define default_interval inside the Hash yet be able to referenced by other field so that the default_interval can be overwritten by other Chef means such as environment json.

That is the reason why I do not choose simply define a global default_interval var.

Currently, I use following definition to represent default_interval

default['mgmt']['query'] = {
    'default_interval' => {'diff' => 3600, 'snapshot' => 86400 * 7},
    'tables' => {
        'deb_packages' => {} # default_interval,
        'rpm_packages' => {} # default_interval,
        ...
    }
}

The empty {} can be overwritten by other chef means such as environment json to change to {diff: 600, snapshot:86400}

Any better cool way?

I have changed all of your tags to get you into a better visibility location since this question is very chef specific — engineersmnky, Mar 30 '18 at 13:13
removed the ruby tag, this a chef-specific question about the node object and not a ruby Hash object — lamont, Apr 06 '18 at 15:46
@lamont Although I am using these code in chef, but all chef code is actually Ruby code! I have confirmed that this question and answer can be applied to non-chef Ruby environment, so would you please re-add the ruby tag? — osexp2000, Apr 09 '18 at 00:52
The node object that you are manipulating is very specific to the chef environment and you are getting answers to this question which very clearly mistake it for a Hash, which is is not. It doesn't even inherit from Hash and `Chef::Node < Hash` as an expression is falsey. — lamont, Apr 13 '18 at 02:38

score 1 · Accepted Answer · answered Mar 30 '18 at 13:24

1

That is the reason why I do not choose simply define a global default_interval var.

No need, you can define a local variable.

default['mgmt']['query'] = {
    'default_interval' => (default_interval = {'diff' => 3600, 'snapshot' => 86400 * 7}),
    'tables' => {
        'deb_packages' => default_interval,
        'rpm_packages' => default_interval,
    }
}

This style is not very common in ruby and is guaranteed to raise a few eyebrows, but it does what you want.

answered Mar 30 '18 at 13:24

Sergio Tulentsev

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Thank you! This is what I want. But unfortunately, as I tested, the `default_interval` is still treated as a global variable. I call `puts default_interval` after this statement, it print out "{"diff"=>3600, "snapshot"=>604800}", maybe it is a chef specific result. Anyway, this is the most elegant way I ever know. – osexp2000 Apr 04 '18 at 03:32
@osexp2003: no, it's not global, it's local to the scope. Now if you don't have any method or class definitions, the scope is likely the whole rest of the file. – Sergio Tulentsev Apr 04 '18 at 07:14
You are right. I'v confirmed that the variable is file-level. So no problem now. thank you again. – osexp2000 Apr 05 '18 at 01:29
1

you can write more conventional code by just defining the local variable above the whole codeblock, and then using it in all three places. – lamont Apr 13 '18 at 02:35

score -1 · Answer 2 · edited Mar 28 '18 at 11:32

-1

Declare node as

node = Hash.new { |h, k| h[k] = {} }

and your code will just be executed as is, producing a structure you need. The latter is named “hash” in ruby btw.

More info on Hash#new accepting a block.

edited Mar 28 '18 at 11:32

SRack

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answered Mar 28 '18 at 07:59

Aleksei Matiushkin

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Excuse me what do you mean “interesting”? Have you expected some more php-ish answer, like 20 lines of code? – Aleksei Matiushkin Mar 28 '18 at 08:12
I could not open your link. But it seems useful sometimes. I am just expecting some usage like self['default_interval']. The above code is a piece of chef attributes/default.rb so seems nobody write ruby statement (such as function call or something). – osexp2000 Mar 28 '18 at 08:32
I found a description at https://docs.ruby-lang.org/en/2.0.0/Hash.html, it is a cool field getter call. I will try it – osexp2000 Mar 28 '18 at 08:37
@osexp2003 are you saying `node` is a `Chef::Node`? That is slightly different than a ruby `Hash`. While it does define a `#[]` method this method is actually delegated to `attributes` which is closer to a `Hash` but not quite (it's a `Mash` *Chef's subclass of ruby's native `Hash`) I think what you are actually looking for is a simple as `def default_interval; {'diff' => 3600, 'snapshot' => 86400 * 7}; end` then your references will work as method calls to that method. – engineersmnky Mar 29 '18 at 19:07
@engineersmnky thank you. I'v added more description to the question. Actually the `node` should be `default` which might still be slightly different as ruby `Hash`? Yes, I am looking for a answer let me define field default_interval yet can be referenced by another field. – osexp2000 Mar 30 '18 at 03:39
Yeah node in this context is a Chef::Node object which is something that smells a bit like a Hash but is overly complicated, and already supports autovivification and converting symbol keys to strings, along with a somewhat overly complicated multiple precedence level structure. – lamont Apr 06 '18 at 15:49

Chef node attribute definition: define a field by reference other field

2 Answers2