Скачать или смотреть How to Assign List dynamic to List Khana in Flutter-Firestore

Learn how to successfully convert a `List dynamic ` from Firestore to a `List Khana ` model in Flutter, and resolve common data type errors.
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This video is based on the question https://stackoverflow.com/q/75039321/ asked by the user 'Mohammad Abdur Rehman Cheema' ( https://stackoverflow.com/u/14177764/ ) and on the answer https://stackoverflow.com/a/75041900/ provided by the user 'krishnaacharyaa' ( https://stackoverflow.com/u/13431819/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Introduction

If you're working with Flutter and Firestore, you may encounter various issues when fetching data from Firestore—the error message can be particularly frustrating. One common issue developers face is trying to assign a List<dynamic> from Firestore to a more specific list type, such as List<Khana>.

In this guide, we will go through a scenario involving the Orders class that contains a list of Khana objects. We'll examine how to properly convert the data fetched from Firestore into the desired list type while resolving the error: “Expected a value of type List Khana , but got one of type List dynamic ”.

Understanding the Problem

Context of the Error

When fetching data from Firestore, the data type returned is often a collection of dynamic types, hence the List<dynamic>. In your case, the khana field within the Orders class is expected to be a List<Khana>, which leads to a type mismatch and subsequently, an error.

Analyzing the Code

You may have structured your fetchAndSetOrder function correctly, but ensuring that the data fetched can be correctly cast to the intended type is key. The constructor Orders.fromFirestore pulls the khana data directly from the fetched document, which causes the type miss when assigned.

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The Khana Model

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The objective is to convert the List<dynamic> received from Firestore to a List<Khana> to maintain type safety and leverage the model’s attributes effectively.

Solution

Using Dart's List Conversion

The key to resolving the error lies in using Dart's capability to convert lists. We’ll utilize the map function along with a factory constructor to transform each dynamic item in the fetched list into an instance of Khana.

Here’s How to Do It

Modify the Orders.fromFirestore constructor as follows:

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Breakdown of the Code

Data Extraction: You first extract the data from the document snapshot as a LinkedHashMap<String, dynamic>.

Mapping to Khana: The key part is List<Khana>.from(data['khana'].map((x) => Khana.fromJson(x))). This line:

Uses map to iterate over each item in data['khana'].

Converts each item (which is in JSON format) to a Khana instance using the fromJson method.

Finally, wraps the result into a List<Khana>.

Conclusion

By adjusting the way you handle the data coming from Firestore, specifically using Dart's list mapping capabilities, you can effectively resolve data type errors when working with Firestore and specific model classes in Flutter.

This small yet vital modification allows you to work with strong-typed lists, reducing errors and increasing code readability and maintainability.

If you ever find yourself in a similar situation, remember this structured approach to convert your List<dynamic> to your desired list type.