read-simple-json.md

title	timestamp	author	published	description	tags	todo
Read simple JSON and deserialize into a struct	2023-12-01 03:30:01 -0800	szabgab	true	Deserializing a JSON string is not too hard especially if the JSON is simple and if we don't need all the fields.	JSON Deserialize serde serde_json Debug dbg! assert! assert_eq!	TODO look at the bottom of this page.

In another post we saw how to process arbitrary JSON string without defining anything about it and without creating a struct. It can be a good idea when we start experimenting with a new JSON structure, but eventually we will want to create a struct and deserialize the JSON string into this struct.

In this example we start on that journey for a simple JSON File that looks like this:

{% include file="examples/read-simple-json/data.json" %}

We need both serde and serde_json

For this we'll need both serde_json and serde with the derive feature.

{% include file="examples/read-simple-json/Cargo.toml" %}

The code

{% include file="examples/read-simple-json/src/main.rs" %}

We need to create a struct to represent the data where we define the expected fields and their type.

As our real data does not have a lot of fields we could have created a struct defining all the fields, but I wanted to show the process you might take if you have a bigger JSON file and don't want to do all the work up-front, or if you don't actually need all the fields from the JSON string.

struct Person {
    fname: String,
    married: bool,
}

We need to add the Deserialize trait to it. I also included the Debug trait to allow us to use the dbg! macro to display the content of the struct.

#[derive(Deserialize, Debug)]

The we open the file and use the serde_json::from_reader function to read in the file.

serde_json::from_reader(&file).expect("JSON parsing error")

The important part that differs from the generic JSON parsing example is that we read the data into a variable that was annotated with the Person struct:

let data: Person =

The we can use the dbg! macro to show the content of the struct. We also use the assert_eq! to verify the value of a string and the assert! macro to verify the value of a bool (boolean) field.

Conclusion

It is quite easy to get started deserializing a simple JSON file, especially if we don't need to get all the fields right at the beginning.

There are, however many more aspect of JSON we need to deal with.

• How to handle more complex JSON structures?
• How to handle a JSON that has a list (array) at the root?
• How can we make sure we mapped all the fields?
• What to do if a field we added to the struct is missing from the JSON?
• What to do if there is a typo in the fields of the JSON?