bx-orm: Entity Relationships

Relationship Overview

Every relationship property uses fieldtype to declare its kind.

One-to-One

// Contact.bx — has one Address
class persistent="true" entityName="Contact" {

    property name="id"      fieldtype="id" generator="native";
    property name="name"    type="string";

    // One-to-one: each Contact has exactly one Address
    property name="address"
        fieldtype = "one-to-one"
        class     = "Address";

}

Set lazy="false" to eagerly load the related entity:

property name="address"
    fieldtype = "one-to-one"
    class     = "Address"
    lazy      = "false";

One-to-Many

// User.bx — has many Posts
class persistent="true" entityName="User" {

    property name="id"    fieldtype="id" generator="native";
    property name="email" type="string";

    // One-to-many: a User has many Posts
    property name="posts"
        fieldtype   = "one-to-many"
        class       = "Post"
        fkcolumn    = "author_id"   // FK column in the posts table
        lazy        = "true"
        singularName= "post";       // enables addPost(), removePost()

}

Accessing the collection:

user = entityLoadByPK( "User", 1 )
posts = user.getPosts()       // returns array of Post entities
user.addPost( newPost )       // uses singularName
user.removePost( oldPost )    // uses singularName
user.hasPosts()               // boolean

Many-to-One

// Post.bx — belongs to one Author (User)
class persistent="true" entityName="Post" {

    property name="id"    fieldtype="id" generator="native";
    property name="title" type="string";

    // Many-to-one: many Posts share one Author
    property name="author"
        fieldtype = "many-to-one"
        class     = "User"
        fkcolumn  = "author_id";

}

post   = entityLoadByPK( "Post", 42 )
author = post.getAuthor()     // returns User entity

Many-to-Many

// Author.bx — can write many Books; Books can have many Authors
class persistent="true" entityName="Author" {

    property name="id"   fieldtype="id" generator="native";
    property name="name" type="string";

    property name="books"
        fieldtype   = "many-to-many"
        class       = "Book"
        linktable   = "author_books"   // join table
        fkcolumn    = "author_id"      // this entity's FK in join table
        inversejoin = "book_id"        // other entity's FK in join table
        singularName= "book";

}

// Book.bx — inverse side
class persistent="true" entityName="Book" {

    property name="id"      fieldtype="id" generator="native";
    property name="title"   type="string";

    property name="authors"
        fieldtype   = "many-to-many"
        class       = "Author"
        linktable   = "author_books"
        fkcolumn    = "book_id"
        inversejoin = "author_id"
        singularName= "author";

}

Relationship Property Annotations

Cascade Options

// Cascade save/delete to child records
property name="posts"
    fieldtype = "one-to-many"
    class     = "Post"
    fkcolumn  = "user_id"
    cascade   = "all";         // save, update, and delete cascade

// Cascade save only (not delete)
property name="address"
    fieldtype = "one-to-one"
    class     = "Address"
    cascade   = "save-update";

Lazy vs Eager Loading

// Lazy (default) — related entity loaded only when accessed
property name="posts" fieldtype="one-to-many" class="Post" lazy="true";

// Eager — loads related entity in the same SQL query
property name="address" fieldtype="one-to-one" class="Address" lazy="false";

Recommendation: Use lazy="true" (default) for collections (one-to-many, many-to-many) and consider lazy="false" only for single-record relationships (one-to-one) where you always need the related data.

N+1 Problem Prevention

Use batchSize to batch-load related entities:

// Entity-level batchSize
class persistent="true" entityName="Post" batchSize="20" {
    // ...
}

// Relationship-level batchSize
property name="comments"
    fieldtype = "one-to-many"
    class     = "Comment"
    fkcolumn  = "post_id"
    batchSize = "20";

Common Pitfalls

• ❌ Do NOT forget fkcolumn on one-to-many and many-to-one — Hibernate cannot infer it
• ❌ Do NOT use lazy="false" on large collections — will load thousands of records
• ❌ Do NOT forget linktable on many-to-many — it is required
• ✅ Always set singularName on collection relationships for clean addX() / removeX() API
• ✅ Use cascade="all-delete-orphan" on owned one-to-many collections (e.g., order → line items)
• ✅ Define both sides of many-to-many relationships with matching linktable values