byName function to select column value from a json dataset in dataflow

Question

Hi,

I have a dataset with json data as source in a dataflow and want to add a derived column to the stream that will be calculated like this

concat('/folder/',toString(byName('attribute')),'.json')

This works fine

But it my attribute is nested within a json object it does not work... for example

concat('/folder/',toString(byName('parentObject.attribute')),'.json')

If i turn schema drift ,concat('/folder/',parentObject.attribute,'.json') will work fine but this is not what i need since it needs to be dynamic.

On more remark : toString(byName('parentObject') would return the json string of the inner object but how to parse and get only 'attribute' ?

Thanks for the help

Answer 1

You should use byItem. Either -

toString(byItem( byName('parentObject'), 'attribute'))

or

byItem( byName('parentObject'), 'attribute') ? string

Answer 2

Hello @Victor Brito and welcome to Microsoft Q&A. Thank you for the excellent question. I believe I have an answer for you. Please let me share my steps.

It sounds like you are passing in a JSON object which includes metadata. To stand in for this, I created a parameter, and gave it the string literal

'{"ParentObject":{"foo":"bar","attribute":"thing_I_want"},"otherObject":4}'  

While trying to access the JSON as an object is the commonsense approach, since it isn't working, I decided another approach was in order.

Since JSON is fundamentally just well-formatted text, I figured a regular expression should be able to extract what we need.

byName(regexExtract($parameter1,`"attribute":"(\w+)"`))  

I tested this by adding a column named "thing_I_want" , and placing the above in a later derived column1.