Working with Relations - GDScript 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

var collection = await pb.collections.get_one("posts")

collection.fields.append({
    "name": "user",
    "type": "relation",
    "options": {
        "collectionId": "users"  # ID of related collection
    },
    "maxSelect": 1,  # Single relation
    "required": true
})

# Multiple relation field
collection.fields.append({
    "name": "tags",
    "type": "relation",
    "options": {
        "collectionId": "tags"
    },
    "maxSelect": 10,  # Multiple relation (max 10)
    "minSelect": 1    # Minimum 1 required
})

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

Creating Records with Relations

Single Relation

# Create a post with a single user relation
var post = await pb.collection("posts").create({
    "title": "My Post",
    "user": "USER_ID"
})

Multiple Relations

# Create a post with multiple tags
var 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
var 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+": "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": "TAG_ID"  # 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"
})

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

Complete Example

# Get existing post
var post = await pb.collection("posts").get_one("POST_ID")
print(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
})

var updated = await pb.collection("posts").get_one("POST_ID")
print(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
var comment = await pb.collection("comments").get_one("COMMENT_ID", {
    "expand": "user"
})

print(comment.get("expand", {}).get("user", {}).get("name"))  # "John Doe"
print(comment.user)  # Still the ID: "USER_ID"

Expand Multiple Relations

# Expand multiple relations (comma-separated)
var comment = await pb.collection("comments").get_one("COMMENT_ID", {
    "expand": "user,post"
})

print(comment.get("expand", {}).get("user", {}).get("name"))   # "John Doe"
print(comment.get("expand", {}).get("post", {}).get("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
var comment = await pb.collection("comments").get_one("COMMENT_ID", {
    "expand": "user,post.tags"
})

# Access nested expands
var post_expand = comment.get("expand", {}).get("post", {})
print(post_expand.get("expand", {}).get("tags", []))
# Array of tag records

# Expand even deeper
var post = await pb.collection("posts").get_one("POST_ID", {
    "expand": "user,comments.user"
})

# Access: post.get("expand", {}).get("comments", [])[0].get("expand", {}).get("user", {})

Expand with List Requests

# List comments with expanded users
var comments = await pb.collection("comments").get_list(1, 20, {
    "expand": "user"
})

for comment in comments.items:
    print(comment.message)
    var user = comment.get("expand", {}).get("user", {})
    if user.has("name"):
        print(user.name)

Expand Single vs Multiple Relations

# Single relation - expand.user is a dictionary
var post = await pb.collection("posts").get_one("POST_ID", {
    "expand": "user"
})
var user_expand = post.get("expand", {}).get("user", {})
print(user_expand is Dictionary)  # true

# Multiple relation - expand.tags is an array
var post_with_tags = await pb.collection("posts").get_one("POST_ID", {
    "expand": "tags"
})
var tags_expand = post_with_tags.get("expand", {}).get("tags", [])
print(tags_expand is Array)  # 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 null or missing
var comment = await pb.collection("comments").get_one("COMMENT_ID", {
    "expand": "user"
})

var user_expand = comment.get("expand", {}).get("user", null)
if user_expand != null:
    print(user_expand.get("name", ""))
else:
    print("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
var post = await pb.collection("posts").get_one("POST_ID", {
    "expand": "comments_via_post"
})

# comments_via_post is always an array (even if original field is single)
print(post.get("expand", {}).get("comments_via_post", []))
# Array of comment records

Back-Relation with Nested Expand

# Get post with comments, and expand each comment's user
var post = await pb.collection("posts").get_one("POST_ID", {
    "expand": "comments_via_post.user"
})

# Access nested expands
var comments = post.get("expand", {}).get("comments_via_post", [])
for comment in comments:
    print(comment.get("message", ""))
    var user = comment.get("expand", {}).get("user", {})
    if user.has("name"):
        print(user.name)

Filtering with Back-Relations

# List posts that have at least one comment containing "hello"
var posts = await pb.collection("posts").get_list(1, 20, {
    "filter": "comments_via_post.message ?~ \"hello\"",
    "expand": "comments_via_post.user"
})

for post in posts.items:
    print(post.get("title", ""))
    var comments = post.get("expand", {}).get("comments_via_post", [])
    for comment in comments:
        var message = comment.get("message", "")
        var user = comment.get("expand", {}).get("user", {})
        var user_name = user.get("name", "Unknown")
        print("  - " + message + " by " + user_name)

Sorting with Back-Relations

# Sort posts by number of comments (requires aggregation or custom logic)
var posts = await pb.collection("posts").get_list(1, 20, {
    "expand": "comments_via_post",
    "sort": "-created"
})

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
var post = await pb.collection("posts").get_one("POST_ID", {
    "expand": "comments_via_post"
})

# Always an array
var comments = post.get("expand", {}).get("comments_via_post", [])
print(comments is Array)  # 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)
var post = await pb.collection("posts").get_one("POST_ID")

# Fetch comments separately with pagination
var comments = await pb.collection("comments").get_list(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
var 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
var full_post = await pb.collection("posts").get_one(post.id, {
    "expand": "author,tags"
})

print(full_post.get("title", ""))
var author = full_post.get("expand", {}).get("author", {})
print("Author: ", author.get("name", ""))

print("Tags:")
var tags = full_post.get("expand", {}).get("tags", [])
for tag in tags:
    print("  - ", tag.get("name", ""))

Example 2: Comment System with Nested Relations

# Create a comment on a post
var comment = await pb.collection("comments").create({
    "message": "Great article!",
    "post": "POST_ID",
    "user": "USER_ID"
})

# Get post with all comments and their authors
var post = await pb.collection("posts").get_one("POST_ID", {
    "expand": "author,comments_via_post.user"
})

print("Post: ", post.get("title", ""))
var author = post.get("expand", {}).get("author", {})
print("Author: ", author.get("name", ""))

var comments = post.get("expand", {}).get("comments_via_post", [])
print("Comments (", comments.size(), "):")
for comment in comments:
    var user = comment.get("expand", {}).get("user", {})
    print("  ", user.get("name", ""), ": ", comment.get("message", ""))

Example 3: Dynamic Tag Management

class_name PostManager

var pb  # BosBase client

func add_tag(post_id: String, tag_id: String) -> void:
    await pb.collection("posts").update(post_id, {
        "tags+": tag_id
    })

func remove_tag(post_id: String, tag_id: String) -> void:
    await pb.collection("posts").update(post_id, {
        "tags-": tag_id
    })

func set_priority_tags(post_id: String, tag_ids: Array) -> void:
    # Clear existing and set priority tags first
    var post = await pb.collection("posts").get_one(post_id)
    await pb.collection("posts").update(post_id, {
        "tags": tag_ids
    })

func get_post_with_tags(post_id: String) -> Dictionary:
    return await pb.collection("posts").get_one(post_id, {
        "expand": "tags"
    })

# Usage
var manager = PostManager.new()
manager.pb = pb
await manager.add_tag("POST_ID", "NEW_TAG_ID")
var post = await manager.get_post_with_tags("POST_ID")

Example 4: Filtering Posts by Tag

# Get all posts with a specific tag
var posts = await pb.collection("posts").get_list(1, 50, {
    "filter": "tags.id ?= \"TAG_ID\"",
    "expand": "author,tags",
    "sort": "-created"
})

for post in posts.items:
    var title = post.get("title", "")
    var author = post.get("expand", {}).get("author", {})
    var author_name = author.get("name", "Unknown")
    print(title, " by ", author_name)

func get_user_dashboard(user_id: String) -> void:
    # Get user with all related content
    var user = await pb.collection("users").get_one(user_id, {
        "expand": "posts_via_author,comments_via_user.post"
    })
    
    print("Dashboard for ", user.get("name", ""))
    
    var posts = user.get("expand", {}).get("posts_via_author", [])
    print("\nPosts (", posts.size(), "):")
    for post in posts:
        print("  - ", post.get("title", ""))
    
    var comments = user.get("expand", {}).get("comments_via_user", [])
    print("\nRecent Comments:")
    var recent_comments = comments.slice(0, 5)
    for comment in recent_comments:
        var post_expand = comment.get("expand", {}).get("post", {})
        var post_title = post_expand.get("title", "Unknown Post")
        print("  On \"", post_title, "\": ", comment.get("message", ""))

Example 6: Complex Nested Expand

# Get a post with author, tags, comments, comment authors, and comment reactions
var post = await pb.collection("posts").get_one("POST_ID", {
    "expand": "author,tags,comments_via_post.user,comments_via_post.reactions_via_comment"
})

# Access deeply nested data
var comments = post.get("expand", {}).get("comments_via_post", [])
for comment in comments:
    var user = comment.get("expand", {}).get("user", {})
    var user_name = user.get("name", "Unknown")
    print(user_name, ": ", comment.get("message", ""))
    
    var reactions = comment.get("expand", {}).get("reactions_via_comment", null)
    if reactions != null:
        print("  Reactions: ", reactions)

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:

var expand = record.get("expand", {})
var user = expand.get("user", null)
if user != null:
    print(user.get("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