Discarding nulls - jmespath-community/jmespath.spec GitHub Wiki
Problem statement
Discarding null
values is a surprisingly frequently requested feature from JMESPath.
The canonical answer is to use the merge()
function and let external deserializers discard / ignore those values.
That said, the items()
, from_items()
and zip()
functions let you build primitives to achieve this.
How to
Given:
{ "a": 1, "b": "", "c": null }
The following expressions can be useful:
from_items( items(@) [? @[1]!=`null` ] )
returns `` { "a": 1, "b": "" }from_items( items(@) [? @[1]!='' ] )
returns `` { "a": 1, "c": null }from_items( items(@) [? @[1] ] )
returns `` { "a": 1 }
More complex objects
The previous primitives can be used in more complex scenarios where the discarded values are in nested object structures.
Given:
{
"key1": { "a": 1, "b": "", "c": null },
"key2": { "a": 2, "b": "bee", "c": "" }
}
This requires a way to split keys from their values, operate on the values using one of the primitives referred to above, and reconstruct the object.
The following expressions are needed:
keys(@)
to extract the key from the object.values(@)[*].from_items( items(@)[[email protected][1]] )
to discard all falsy values, that include bothnull
and empty strings.
Given those two expressions, the let()
function lets you create a scope to hold both keys and computed values, and operate on them using an expression-type
:
let( { k:keys(@), v:values(@)[*].from_items( items(@)[[email protected][1]] ) }, &… )
The first argument to the let()
function creates a multi-select-hash
scope with two properties, k
and v
that hold the keys from the original input and the newly computed values respectively.
The second argument to the let()
function is an expression-type
that can be used to operate on said scope object.
To reconstruct an object from a given set of keys and values, use the following expression:
from_items( { k:k, v:v }.zip(k, v) )
Note: the
let()
function is currently being discussed and subject to change. However, lexical scoping using some form of a let expression will still be possible.