landoop / json-kcql

Provides support for json projections using the sql like syntax of KCQL

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This is a small utility library allowing you to translate the shape of a JSON document. Let's say we have the following json (A description of a Pizza):

  "ingredients": [
      "name": "pepperoni",
      "sugar": 12.0,
      "fat": 4.4
      "name": "onions",
      "sugar": 1.0,
      "fat": 0.4
  "vegetarian": false,
  "vegan": false,
  "calories": 98,
  "fieldName": "pepperoni"

using the library one can apply to types of queries:

  • to flatten it
  • to retain the structure while cherry-picking and/or rename fields The difference between the two is marked by the withstructure* keyword. If this is missing you will end up flattening the structure. This library is dependant on com.datamountaineer.kcql hence you still have to provide a 'from topic'

Let's take a look at the flatten first. There are cases when you are receiving a nested json and you want to flatten the structure while being able to cherry pick the fields and rename them. Imagine we have the following JSON:

  "name": "Rick",
  "address": {
    "street": {
      "name": "Rock St"
    "street2": {
      "name": "Sunset Boulevard"
    "city": "MtV",
    "state": "CA",
    "zip": "94041",
    "country": "USA"

Applying this SQL like syntax

    address.street.*, as streetName2 
FROM topic

the projected new JSON is:

  "name": "Rick",
  "name_1": "Rock St",
  "streetName2": "Sunset Boulevard"

There are scenarios where you might want to rename fields and maybe reorder them. By applying this SQL like syntax on the Pizza JSON

       name, as fieldName, 
       ingredients.sugar as fieldSugar, 
       calories as cals 
FROM topic 

we end up projecting the first structure into this one:

  "name": "pepperoni",
  "ingredients": [
      "fieldName": "pepperoni",
      "fieldSugar": 12.0,
      "fat": 4.4
      "fieldName": "onions",
      "fieldSugar": 1.0,
      "fat": 0.4
  "cals": 98

Flatten rules

  • you can't flatten a json containing array fields
  • when flattening and the column name has already been used it will get a index appended. For example if field name appears twice and you don't specifically rename the second instance (name as renamedName) the new json will end up containing: name and name_1

How to use it

import JsonKcql._
val json: JsonNode= ...
json.kcql("SELECT name, as streetName FROM topic")

As simple as that!

Query Examples

You can find more examples in the unit tests, however here are a few used:

  • flattening
//rename and only pick fields on first level
SELECT calories as C ,vegan as V ,name as fieldName FROM topic

//Cherry pick fields on different levels in the structure
SELECT name, as streetName FROM topic

//Select and rename fields on nested level
SELECT name, address.street.*, as streetName2 FROM topic
  • retaining the structure
//you can select itself - obviousely no real gain on this
SELECT * FROM topic withstructure 

//rename a field 
SELECT *, name as fieldName FROM topic withstructure

//rename a complex field
SELECT *, ingredients as stuff FROM topic withstructure

//select a single field
SELECT vegan FROM topic withstructure

//rename and only select nested fields
SELECT as fieldName, ingredients.sugar as fieldSugar, ingredients.* FROM topic withstructure

Release Notes

0.1 (2017-03-15)

  • first release


Requires gradle 3.4.1 to build.

To build

gradle compile

To test

gradle test

You can also use the gradle wrapper

./gradlew build

To view dependency trees

gradle dependencies #