Nifi JOLT Transform to remove JSON Object from JSON Array by element value

Question

I'm new to JSON JOLT transformations and I'm not sure I can perform the function I am attempting. I have a JSON Array that contains JSON Objects. I want to identify a boolean or string value in the objects. If that value is true or equal to something I want to remove the entire object from the array. If the value is false I want to do nothing. Is this possible in NiFi using a JOLT Transformation (JoltTransformJSON Processor).

I've tried a couple different shift's and remove's but not much success with the syntax getting it right. Below is the before and after of what I'm looking to accomplish, if possible. I am fine if what I'm looking to do is not possible, I just don't know enough to determine that yet.

Before:

[
  {
    "alpha": "a",
    "beta": "b",
    "omega": "o",
    "exists": true
  },
  {
    "alpha": "1",
    "beta": "2",
    "omega": "3",
    "exists": false
  },
  {
    "alpha": "z",
    "beta": "x",
    "omega": "y",
    "exists": true
  }
]

Some Transform that looks for exist being false and removes or only pulls out exists that equal true

After:

[
  {
    "alpha": "a",
    "beta": "b",
    "omega": "o",
    "exists": true
  },
  {
    "alpha": "z",
    "beta": "x",
    "omega": "y",
    "exists": true
  }
]

Answer 1

You can set those boolean values as keys of the respective objects such as :

[
  { // group by the values of the "exists" attribute
    "operation": "shift",
    "spec": {
      "*": {
        "*": "@1,exists.&1.&" // &1 represents going 1 level up the tree 
                              // to grab the index values for each object 
                              // within the array
                              // & is leaf node's value
      }
    }
  },
  { // pick the objects with the "true" key only 
    "operation": "shift",
    "spec": {
      "true": {
        "*": {
          "*": "[#2].&"
        }
      }
    }
  }
]

the demo on the site https://jolt-demo.appspot.com/ is :