Скачать или смотреть How to Handle JSON Mapping with Nested Models in Flutter

Discover how to properly convert your Flutter models to JSON, especially when dealing with lists of nested models, to avoid common errors.
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This video is based on the question https://stackoverflow.com/q/65780664/ asked by the user 'Andrea Visnenza Andy' ( https://stackoverflow.com/u/2894419/ ) and on the answer https://stackoverflow.com/a/65788682/ provided by the user 'Andrea Visnenza Andy' ( https://stackoverflow.com/u/2894419/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Introduction: The JSON Mapping Challenge in Flutter

As a new Flutter developer, converting your models into JSON for API calls can be a bit daunting. If you’ve encountered an error like type '_InternalLinkedHashMap<String, dynamic>' is not a subtype of type 'String', you’re not alone! This typically happens when you're trying to convert a model containing nested models or lists without properly handling their serialization.

In this guide, we’ll dive deep into how to effectively map your models to JSON in Flutter, focusing on how to deal with lists of sub-models. By the end, you’ll understand how to prevent those frustrating errors and successfully serialize your complex models.

Understanding the Problem

The Error You Encountered

You mentioned experiencing the following error when using the toJson function of your RegistrationRequest model:

[[See Video to Reveal this Text or Code Snippet]]

This error commonly occurs when the serialization process doesn’t accurately transform nested objects into the desired format. In your case, it seems like you're having trouble with the userImages list within the RegistrationRequest model.

Your Model Structure

Here’s a quick overview of your models, which comprise a main model (RegistrationRequest) containing a list of another model (UserImage):

RegistrationRequest: Contains user details and a list of UserImage objects.

UserImage: Describes the properties of each user image.

Solution: Correcting the JSON Serialization

Modifying the fromJson Method

To properly initialize your RegistrationRequest object from a JSON map, you need to ensure that you convert each UserImage entry correctly. Here’s how you can modify your fromJson method:

[[See Video to Reveal this Text or Code Snippet]]

Adjusting the toJson Method

Similarly, you also need to update your toJson method to ensure it converts the list of UserImage objects back into the correct format when serializing the RegistrationRequest:

[[See Video to Reveal this Text or Code Snippet]]

Key Modifications Summary

userImages List Handling: Ensure both fromJson and toJson methods properly iterate over the list of UserImage objects for accurate serialization and deserialization.

Return Correct Types: Make sure to return the correct data types for the model properties, especially with nested structures.

Example of Full Updated Model

Here is your modified RegistrationRequest model, which includes the changes mentioned above:

[[See Video to Reveal this Text or Code Snippet]]

Conclusion

Handling JSON mapping in Flutter can be challenging, especially when working with nested models. By ensuring that you accurately iterate over lists and convert nested objects in both your fromJson and toJson methods, you can avoid many common pitfalls and effectively serialize your data for API interactions.

If you encounter any other issues or have questions about Flutter development, feel free to reach out. Happy coding!