Ruby hash array returning key instead of value?

Question

[1] pry(main)> o.items
=> [{"78"=>"{\"size\"=>\"96\", \"side\"=>[]}"}]

[2] pry(main)> o.items[0]
=> {"78"=>"{\"size\"=>\"96\", \"side\"=>[]}"}

[3] pry(main)> o.items[0]['78']
=> "{\"size\"=>\"96\", \"side\"=>[]}"

[4] pry(main)> o.items[0]['78']["side"]
=> "side"

Isn't line 4 supposed to return an empty array? How come its returning "side"?

EDIT

I found this happens after saving the array of hashes (it is an array of hstore on postgresql).

E.g This returns a hash as intended.

o.items << {78 => {"size" => 1, "side" => []}}
o.items
=> [{"78"=> {"size"=>"1", "side"=>[]}}]

But after saving it

o.save
o.items
=> [{"78"=>"{\"size\"=>\"96\", \"side\"=>[]}"}]

I ended up doing eval(o.items[0]['78']) to convert the string back into a hash before making any changes to the hash and making updates. This seems very unnecessary, are there better options?

@Aetherus That seems to answer the question, you should probably write it up as an answer so it's more visible. — thesecretmaster
– thesecretmaster, Commented Oct 6, 2016 at 23:33
"foobar"["oba"] is "oba", as is "foobar"[/o.a/]. You searched for "side" in a string, and it was found. — Amadan
– Amadan, Commented Oct 7, 2016 at 0:58

roychri · Accepted Answer · 2016-10-07 02:35:49Z

2

As the commenters pointed out, the problem lies in the data type. You thought that the value was a hash when it fact it was a string. The escaped double quotes are the clues here.

"{\"size\"=>\"96\", \"side\"=>[]}"

vs

{"size"=>"96", "side"=>[]}

See the difference?

When you have a string and access it using the String#[] method, it returns that string when it is present. The documentation says:

If a match_str is given, that string is returned if it occurs in the string.

Check this link for more details: http://ruby-doc.org/core-2.3.0/String.html#method-i-5B-5D

answered Oct 7, 2016 at 2:35

roychri

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1 Comment

echan00 Over a year ago

I've added more info about how this happened in my question. It seems like the inside of the hash was converted into a string after saving to my postgresql database. Is the only way to go about this to convert the string back to a hash to make updates to it?

tadman · Accepted Answer · 2016-10-08 03:08:41Z

If this is being saved into a Rails property then you need to handle that sort of data properly. This is best done using one of the following methods:

Using a JSON column type to store arbitrary structures.
Flagging the column with serialize to handle arbitrary data.

MySQL 5.7 and Postgres 9.3 or better support JSON as a column type so I'd steer in that direction if possible. If that's not an option, use a regular TEXT column and the serialize option.

That will save your structure in a format that ActiveRecord will reconstitute back in to its original form. You won't have to deal with the stringified hash.

What you were inadvertently tripping here was the String#[] method which can take a variety of options, but one of which is a substring to match. In your case that Ruby hash-as-string contains the phrase you're looking for, just as this works:

string = "ruby has strings"

string["ruby"]
# => "ruby"

string["test"]
# => nil

The [] method is used for all sorts of things as a matter of convenience, though this particular usage is often overlooked.

WARNING: Using eval to reconstitute a hash from a string is really a last resort option and should be avoided if there's any other way, no matter how awkward or inconvenient it is. Evaluating arbitrary strings is a massive liability as if anything should get into that which could be evaluated as code it's very easy to compromise your system.

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