3. Recursive & Tree Operations

mybatis-dynamic has built-in support for hierarchical data (Adjacency List pattern), such as Categories, Departments, or Menus.

3.1 Recursive List vs. Recursive Tree

• queryRecursiveList: Fetches the hierarchy but returns a flattened list.
• Useful for: Searching, calculations, or when you just need all descendants/ancestors without the nested structure.
• queryRecursiveTree: Fetches the hierarchy and assembles it into a nested tree structure.
• Requirement: Your entity class must have a field to hold children (e.g., List<Node> children).

3.2 Key Concepts

1. initNodeCondition: Defines the "Starting Points" (Roots) of your recursion.
2. recursiveDown:
   • true: Find children, grandchildren, etc. (Downwards).
   • false: Find parents, grandparents, etc. (Upwards).

3.3 Example: Category Tree

Model:

@Model
public class Category {
    @IdField private String id;
    private String parentId;
    private String name;

    // Container for children (not a DB column)
    @IgnoreField
    private List<Category> children;
}

Scenario 1: Fetch Full Tree (Root -> Leaves)

// 1. Define roots: Categories with no parent (parentId is null/empty)
List<Category> tree = categoryService.queryRecursiveTreeChain()
    .initNodeCondition(c -> c.isNull("parentId").or().eq("parentId", ""))
    .exec();

// Result is a List of 'Root' categories, each containing their 'children' recursively.

Scenario 2: Fetch Breadcrumbs (Leaf -> Root)

// 1. Start from a specific sub-category
// 2. recursiveDown(false) implies "Go Up" to find parents
List<Category> breadcrumbs = categoryService.queryRecursiveListChain()
    .initNodeCondition(c -> c.eq("id", "sub-cat-123"))
    .recursiveDown(false) // Look upwards
    .exec();

// Result: [SubCategory, ParentCategory, RootCategory] (Order depends on traversal)

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