hash_dig

HashDig

It's a little tricky getting through deeply-nested JSON data with Crystal, a consequence of stronger types. Since each value can be any of JSON::Type, you can't traverse deeper into the object until you've cast it to a Hash. But this gets ugly.

This class attempts to make it easier dig deeply into the hash by passing a string path. The hash_dig.crsource file has an example of this, which is explained here.

Take a JSON object that gets sent back to the server:

  {
    "id": "abd0jjlkladiu2",
    "name": {
      "first": "Bob",
      "last" : "Smith" },
    "children" : {
        "Tom" : {"age" : 6, "sex" : "male" },
        "Sue" : {"age" : 4, "sex" : "female" } }
  }

That's going to come into the server as a string:

string_data = "{\"id\":\"abd0jjlkladiu2\",\"name\":{\"first\":\"Bob\",\"last\":\"Smith\"},\"children\":{\"Tom\":{\"age\":6,\"sex\":\"male\"},\"Sue\":{\"age\":4,\"sex\":\"female\"}}}\"

The first step is to parse it

JSON.parse string_data    # JSON::Any

But now you're faced with parsing every step along the way; it even has to be cast into a Hash before you can accessing the first key/value pairs. Ultimately, the string_data received is just a delivery vehicle for individual bits of data meaningful to the application. So here's what HashDig does: it recursively traverses the provided path, and if it's able to get all the way to the end of the path, it returns the value it finds. If it can't traverse the entire path, it returns nil.

First, some simple examples, on the first level of the example JSON:

name = HashDig.dig simple_json, "name"    # {"first" => "Bob", "last" => "Smith"}
id   = HashDig.dig simple_json, "id"      # "abd0jjlkladiu2"

How about getting the first name from the data?

first_name = HashDig.dig simple_json, "name,first"  # "Bob"

It's important to note at this point that the compile time type of name, id, and first_name are still JSON::Type; you will need to do a final conversion, so maybe it looks more like this:

name=HashDig.dig(simple_json,"name").as(Hash(String,JSON::Type))  # {"first" => "Bob", "last" => "Smith"}
id=HashDig.dig(simple_json, "id").as(String)        		 # "abd0jjlkladiu2"
first_name=HashDig(simple_json, "name,first").as(String) # "Bob"

Future Improvements

It having to do a final cast at the end is a bit of a pain. In the name example, the resulting hash is Hash(String,String) but it can't be case directly with #as() into this format.

Another area not addressed is arrays. The ability to traverse arrays and other objects by including a means of passing the index in the path would be helpful: HashDig.dig simple_json, "children,2,age"would return the age of the second child.