How to model recursive data structures in graphql?

@denis 

To model recursive data structures in GraphQL, you can use the concept of nested types and a special GraphQL type called the GraphQLList. Here's how you can approach modeling recursive data structures in GraphQL:

1. Define the GraphQL types: Start by defining the GraphQL types that represent your recursive data structure. For example, let's say you have a "Category" type that can have many child categories.

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type Category {
  id: ID!
  name: String!
  children: [Category!]!
}

In this example, the children field is of type [Category!]! which means it is a list of non-null Category objects. This allows for recursion as each Category can have child categories.

1. Create query and mutation types: Define the query and mutation types that allow querying and modifying the recursive data structure.

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type Query {
  getCategories: [Category!]!
  getCategory(id: ID!): Category
}

type Mutation {
  createCategory(name: String!, parentId: ID): Category!
}

In this example, the getCategories field returns a list of all top-level categories, while getCategory retrieves a single category by its ID. The createCategory mutation allows creating new categories and optionally specifying a parent category using the parentId argument.

1. Implement resolvers: Implement the resolvers for the query and mutation fields to fetch and manipulate the data.

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const resolvers = {
  Query: {
    getCategories: () => {
      // Return the top-level categories
    },
    getCategory: (parent, { id }) => {
      // Retrieve a category by id
    },
  },
  Mutation: {
    createCategory: (parent, { name, parentId }) => {
      // Create a new category
    },
  },
  Category: {
    children: (parent) => {
      // Retrieve the children of a category
    },
  },
};

In the Category resolver, you can define the logic to fetch the child categories of a given category by its ID.

1. Handle recursion in resolver: To handle recursion, you can implement the logic in the children resolver to recursively fetch the child categories.

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const resolvers = {
  // ...
  Category: {
    children: (parent) => {
      // Retrieve the children of a category
      const childrenIds = parent.childrenIds; // Assuming you have an array of child IDs
      const children = [];

      for (const childId of childrenIds) {
        const childCategory = getCategory(childId); // Fetch the child category
        childCategory.children = childCategory.children(); // Resolve the child's children recursively
        children.push(childCategory);
      }

      return children;
    },
  },
};

In this example, the children resolver fetches the child IDs from the parent category and recursively resolves each child's children until there are no more child categories, creating the recursive data structure.

That's it! With this approach, you can model and work with recursive data structures in GraphQL efficiently.