This seems like a simple use case. We are trying to obtain the number of items in an array using the JSONPath function length().
The test object is:
{
"array": [1,2,3]
}
The JSONPath that were tested include:
$[length(@.foo)]
and
$.foo.length()
In both cases, the result via the json-everything library test page is "Path is invalid":
We are currently using the Newtonsoft.Json library which does not seem to support JSONPath aggregate or filtering functions.
We were encouraged to try the json-everything library thank to this comment on GitHub which claims that length() is supported.
The json-everything release notes shows that this function should be supported as of version 0.30.
Thanks for your help.
According to the spec, all functions (including length()) are only valid inside filter expressions. There are some issues (that will be further discussed if a JSON Path 2.0 is ever developed) that discuss using a function outside of filter expressions, but that didn't make it into the spec's first release.
Because of that, expressions like $[length(@.foo)] and $.foo.length() are invalid.
A valid use of the length() function is something like $.foo[?length(@) > 5], which would find all children of foo which themselves have more than 5 children.
Extending your example,
{
"array": [1,2,3],
"obj": { "foo": 1, "bar": 2, "baz": 3 },
"longer": [1,2,3,4,5],
"not-array": 3
}
the path $[?length(@) == 3] would select the nodes at $.array and $.obj because their values are containers with 3 items, but it wouldn't select $.longer or $.not-array because their values aren't containers with 3 items.
My suggestion for $[length(@.foo)] is to use a negative index: $[-1], and for $.foo.length(), use $.foo and just count the results in code.