Working with Relations - JavaScript SDK Documentation

Overview

Relations allow you to link records between collections. BosBase supports both single and multiple relations, and provides powerful features for expanding related records and working with back-relations.

Key Features:

Single and multiple relations
Expand related records without additional requests
Nested relation expansion (up to 6 levels)
Back-relations for reverse lookups
Field modifiers for append/prepend/remove operations

Relation Field Types:

Single Relation: Links to one record (MaxSelect <= 1)
Multiple Relation: Links to multiple records (MaxSelect > 1)

Backend Behavior:

Relations are stored as record IDs or arrays of IDs
Expand only includes relations the client can view (satisfies View API Rule)
Back-relations use format: collectionName_via_fieldName
Back-relation expand limited to 1000 records per field

Setting Up Relations

Creating a Relation Field

const collection = await pb.collections.getOne('posts');

collection.fields.push({
  name: 'user',
  type: 'relation',
  collectionId: 'users',  // ID of related collection
  maxSelect: 1,           // Single relation
  required: true
});

// Multiple relation field
collection.fields.push({
  name: 'tags',
  type: 'relation',
  collectionId: 'tags',
  maxSelect: 10,          // Multiple relation (max 10)
  minSelect: 1,           // Minimum 1 required
  cascadeDelete: false    // Don't delete post when tags deleted
});

await pb.collections.update('posts', { fields: collection.fields });

Creating Records with Relations

Single Relation

// Create a post with a single user relation
const post = await pb.collection('posts').create({
  title: 'My Post',
  user: 'USER_ID'  // Single relation ID
});

Multiple Relations

// Create a post with multiple tags
const post = await pb.collection('posts').create({
  title: 'My Post',
  tags: ['TAG_ID1', 'TAG_ID2', 'TAG_ID3']  // Array of IDs
});

Mixed Relations

// Create a comment with both single and multiple relations
const comment = await pb.collection('comments').create({
  message: 'Great post!',
  post: 'POST_ID',        // Single relation
  user: 'USER_ID',        // Single relation
  tags: ['TAG1', 'TAG2']  // Multiple relation
});

Updating Relations

Replace All Relations

// Replace all tags
await pb.collection('posts').update('POST_ID', {
  tags: ['NEW_TAG1', 'NEW_TAG2']
});

Append Relations (Using + Modifier)

// Append tags to existing ones
await pb.collection('posts').update('POST_ID', {
  'tags+': 'NEW_TAG_ID'  // Append single tag
});

// Append multiple tags
await pb.collection('posts').update('POST_ID', {
  'tags+': ['TAG_ID1', 'TAG_ID2']  // Append multiple tags
});

Prepend Relations (Using + Prefix)

// Prepend tags (tags will appear first)
await pb.collection('posts').update('POST_ID', {
  '+tags': 'PRIORITY_TAG'  // Prepend single tag
});

// Prepend multiple tags
await pb.collection('posts').update('POST_ID', {
  '+tags': ['TAG1', 'TAG2']  // Prepend multiple tags
});

Remove Relations (Using - Modifier)

// Remove single tag
await pb.collection('posts').update('POST_ID', {
  'tags-': 'TAG_ID_TO_REMOVE'
});

// Remove multiple tags
await pb.collection('posts').update('POST_ID', {
  'tags-': ['TAG1', 'TAG2']
});

Complete Example

// Get existing post
const post = await pb.collection('posts').getOne('POST_ID');
console.log(post.tags);  // ['tag1', 'tag2']

// Remove one tag, add two new ones
await pb.collection('posts').update('POST_ID', {
  'tags-': 'tag1',           // Remove
  'tags+': ['tag3', 'tag4']  // Append
});

const updated = await pb.collection('posts').getOne('POST_ID');
console.log(updated.tags);  // ['tag2', 'tag3', 'tag4']

Expanding Relations

The expand parameter allows you to fetch related records in a single request, eliminating the need for multiple API calls.

Basic Expand

// Get comment with expanded user
const comment = await pb.collection('comments').getOne('COMMENT_ID', {
  expand: 'user'
});

console.log(comment.expand.user.name);  // "John Doe"
console.log(comment.user);              // Still the ID: "USER_ID"

Expand Multiple Relations

// Expand multiple relations (comma-separated)
const comment = await pb.collection('comments').getOne('COMMENT_ID', {
  expand: 'user,post'
});

console.log(comment.expand.user.name);   // "John Doe"
console.log(comment.expand.post.title);  // "My Post"

Nested Expand (Dot Notation)

You can expand nested relations up to 6 levels deep using dot notation:

// Expand post and its tags, and user
const comment = await pb.collection('comments').getOne('COMMENT_ID', {
  expand: 'user,post.tags'
});

// Access nested expands
console.log(comment.expand.post.expand.tags);
// Array of tag records

// Expand even deeper
const post = await pb.collection('posts').getOne('POST_ID', {
  expand: 'user,comments.user'
});

// Access: post.expand.comments[0].expand.user

Expand with List Requests

// List comments with expanded users
const comments = await pb.collection('comments').getList(1, 20, {
  expand: 'user'
});

comments.items.forEach(comment => {
  console.log(comment.message);
  console.log(comment.expand.user.name);
});

Expand Single vs Multiple Relations

// Single relation - expand.user is an object
const post = await pb.collection('posts').getOne('POST_ID', {
  expand: 'user'
});
console.log(typeof post.expand.user);  // "object"

// Multiple relation - expand.tags is an array
const postWithTags = await pb.collection('posts').getOne('POST_ID', {
  expand: 'tags'
});
console.log(Array.isArray(postWithTags.expand.tags));  // true

Expand Permissions

Important: Only relations that satisfy the related collection’s viewRule will be expanded. If you don’t have permission to view a related record, it won’t appear in the expand.

// If you don't have view permission for user, expand.user will be undefined
const comment = await pb.collection('comments').getOne('COMMENT_ID', {
  expand: 'user'
});

if (comment.expand?.user) {
  console.log(comment.expand.user.name);
} else {
  console.log('User not accessible or not found');
}

Back-Relations

Back-relations allow you to query and expand records that reference the current record through a relation field.

Back-Relation Syntax

The format is: collectionName_via_fieldName

collectionName: The collection that contains the relation field
fieldName: The name of the relation field that points to your record

Example: Posts with Comments

// Get a post and expand all comments that reference it
const post = await pb.collection('posts').getOne('POST_ID', {
  expand: 'comments_via_post'
});

// comments_via_post is always an array (even if original field is single)
console.log(post.expand.comments_via_post);
// Array of comment records

Back-Relation with Nested Expand

// Get post with comments, and expand each comment's user
const post = await pb.collection('posts').getOne('POST_ID', {
  expand: 'comments_via_post.user'
});

// Access nested expands
post.expand.comments_via_post.forEach(comment => {
  console.log(comment.message);
  console.log(comment.expand.user.name);
});

Filtering with Back-Relations

// List posts that have at least one comment containing "hello"
const posts = await pb.collection('posts').getList(1, 20, {
  filter: "comments_via_post.message ?~ 'hello'",
  expand: 'comments_via_post.user'
});

posts.items.forEach(post => {
  console.log(post.title);
  post.expand.comments_via_post.forEach(comment => {
    console.log(`  - ${comment.message} by ${comment.expand.user.name}`);
  });
});

Sorting with Back-Relations

// Sort posts by number of comments (requires aggregation or custom logic)
const posts = await pb.collection('posts').getList(1, 20, {
  expand: 'comments_via_post',
  sort: '-created'  // Sort by creation date
});

Back-Relation Caveats

Always Multiple: Back-relations are always treated as arrays, even if the original relation field is single. This is because one record can be referenced by multiple records.

// Even if comments.post is single, comments_via_post is always an array
const post = await pb.collection('posts').getOne('POST_ID', {
  expand: 'comments_via_post'
});

// Always an array
console.log(Array.isArray(post.expand.comments_via_post));  // true

UNIQUE Index Exception: If the relation field has a UNIQUE index constraint, the back-relation will be treated as a single object (not an array).
```
// If there's a UNIQUE constraint, it might be single
// Check your schema to confirm
```

1000 Record Limit: Back-relation expand is limited to 1000 records per field. For larger datasets, use separate paginated requests:

// Instead of expanding all comments (if > 1000)
const post = await pb.collection('posts').getOne('POST_ID');

// Fetch comments separately with pagination
const comments = await pb.collection('comments').getList(1, 100, {
  filter: `post = "${post.id}"`,
  expand: 'user',
  sort: '-created'
});

Complete Examples

Example 1: Blog Post with Author and Tags

// Create a blog post with relations
const post = await pb.collection('posts').create({
  title: 'Getting Started with BosBase',
  content: 'Lorem ipsum...',
  author: 'AUTHOR_ID',           // Single relation
  tags: ['tag1', 'tag2', 'tag3'] // Multiple relation
});

// Retrieve with all relations expanded
const fullPost = await pb.collection('posts').getOne(post.id, {
  expand: 'author,tags'
});

console.log(fullPost.title);
console.log(`Author: ${fullPost.expand.author.name}`);
console.log('Tags:');
fullPost.expand.tags.forEach(tag => {
  console.log(`  - ${tag.name}`);
});

Example 2: Comment System with Nested Relations

// Create a comment on a post
const comment = await pb.collection('comments').create({
  message: 'Great article!',
  post: 'POST_ID',
  user: 'USER_ID'
});

// Get post with all comments and their authors
const post = await pb.collection('posts').getOne('POST_ID', {
  expand: 'author,comments_via_post.user'
});

console.log(`Post: ${post.title}`);
console.log(`Author: ${post.expand.author.name}`);
console.log(`Comments (${post.expand.comments_via_post.length}):`);
post.expand.comments_via_post.forEach(comment => {
  console.log(`  ${comment.expand.user.name}: ${comment.message}`);
});

Example 3: Dynamic Tag Management

class PostManager {
  async addTag(postId, tagId) {
    await pb.collection('posts').update(postId, {
      'tags+': tagId
    });
  }

  async removeTag(postId, tagId) {
    await pb.collection('posts').update(postId, {
      'tags-': tagId
    });
  }

  async setPriorityTags(postId, tagIds) {
    // Clear existing and set priority tags first
    const post = await pb.collection('posts').getOne(postId);
    await pb.collection('posts').update(postId, {
      tags: tagIds,
      'tags+': post.tags.filter(id => !tagIds.includes(id))
    });
  }

  async getPostWithTags(postId) {
    return await pb.collection('posts').getOne(postId, {
      expand: 'tags'
    });
  }
}

// Usage
const manager = new PostManager();
await manager.addTag('POST_ID', 'NEW_TAG_ID');
const post = await manager.getPostWithTags('POST_ID');

Example 4: Filtering Posts by Tag

// Get all posts with a specific tag
const posts = await pb.collection('posts').getList(1, 50, {
  filter: 'tags.id ?= "TAG_ID"',
  expand: 'author,tags',
  sort: '-created'
});

posts.items.forEach(post => {
  console.log(`${post.title} by ${post.expand.author.name}`);
});

async function getUserDashboard(userId) {
  // Get user with all related content
  const user = await pb.collection('users').getOne(userId, {
    expand: 'posts_via_author,comments_via_user.post'
  });

  console.log(`Dashboard for ${user.name}`);
  console.log(`\nPosts (${user.expand.posts_via_author.length}):`);
  user.expand.posts_via_author.forEach(post => {
    console.log(`  - ${post.title}`);
  });

  console.log(`\nRecent Comments:`);
  user.expand.comments_via_user.slice(0, 5).forEach(comment => {
    console.log(`  On "${comment.expand.post.title}": ${comment.message}`);
  });
}

Example 6: Complex Nested Expand

// Get a post with author, tags, comments, comment authors, and comment reactions
const post = await pb.collection('posts').getOne('POST_ID', {
  expand: 'author,tags,comments_via_post.user,comments_via_post.reactions_via_comment'
});

// Access deeply nested data
post.expand.comments_via_post.forEach(comment => {
  console.log(`${comment.expand.user.name}: ${comment.message}`);
  if (comment.expand.reactions_via_comment) {
    console.log(`  Reactions: ${comment.expand.reactions_via_comment.length}`);
  }
});

Best Practices

Use Expand Wisely: Only expand relations you actually need to reduce response size and improve performance.

Handle Missing Expands: Always check if expand data exists before accessing:

if (record.expand?.user) {
  console.log(record.expand.user.name);
}

Pagination for Large Back-Relations: If you expect more than 1000 related records, fetch them separately with pagination.
Cache Expansion: Consider caching expanded data on the client side to reduce API calls.
Error Handling: Handle cases where related records might not be accessible due to API rules.
Nested Limit: Remember that nested expands are limited to 6 levels deep.

Performance Considerations

Expand Cost: Expanding relations doesn’t require additional round trips, but increases response payload size
Back-Relation Limit: The 1000 record limit for back-relations prevents extremely large responses
Permission Checks: Each expanded relation is checked against the collection’s viewRule
Nested Depth: Limit nested expands to avoid performance issues (max 6 levels supported)

Collections - Collection and field configuration
API Rules and Filters - Filtering and querying related records