Working with Relations - Elixir 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

alias Bosbase.Client

pb = Client.new("http://127.0.0.1:8090")

# Get collection
{:ok, collection} = Bosbase.collections()
  |> Bosbase.CollectionService.get_one(pb, "posts")

# Add single relation field
new_fields = collection["fields"] ++ [
  %{
    "name" => "user",
    "type" => "relation",
    "collectionId" => "users",  # ID of related collection
    "maxSelect" => 1,           # Single relation
    "required" => true
  }
]

# Add multiple relation field
updated_fields = new_fields ++ [
  %{
    "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
  }
]

# Update collection
{:ok, _updated} = Bosbase.collections()
  |> Bosbase.CollectionService.update(pb, "posts", %{"fields" => updated_fields})

Creating Records with Relations

Single Relation

# Create a post with a single user relation
{:ok, post} = Client.collection(pb, "posts")
  |> Bosbase.RecordService.create(%{
    "title" => "My Post",
    "user" => "USER_ID"  # Single relation ID
  })

Multiple Relations

# Create a post with multiple tags
{:ok, post} = Client.collection(pb, "posts")
  |> Bosbase.RecordService.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
{:ok, comment} = Client.collection(pb, "comments")
  |> Bosbase.RecordService.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
{:ok, _updated} = Client.collection(pb, "posts")
  |> Bosbase.RecordService.update("POST_ID", %{
    "tags" => ["NEW_TAG1", "NEW_TAG2"]
  })

Append Relations (Using + Modifier)

# Append tags to existing ones
{:ok, _updated} = Client.collection(pb, "posts")
  |> Bosbase.RecordService.update("POST_ID", %{
    "tags+" => "NEW_TAG_ID"  # Append single tag
  })

# Append multiple tags
{:ok, _updated} = Client.collection(pb, "posts")
  |> Bosbase.RecordService.update("POST_ID", %{
    "tags+" => ["TAG_ID1", "TAG_ID2"]  # Append multiple tags
  })

Prepend Relations (Using + Prefix)

# Prepend tags (tags will appear first)
{:ok, _updated} = Client.collection(pb, "posts")
  |> Bosbase.RecordService.update("POST_ID", %{
    "+tags" => "PRIORITY_TAG"  # Prepend single tag
  })

# Prepend multiple tags
{:ok, _updated} = Client.collection(pb, "posts")
  |> Bosbase.RecordService.update("POST_ID", %{
    "+tags" => ["TAG1", "TAG2"]  # Prepend multiple tags
  })

Remove Relations (Using - Modifier)

# Remove single tag
{:ok, _updated} = Client.collection(pb, "posts")
  |> Bosbase.RecordService.update("POST_ID", %{
    "tags-" => "TAG_ID_TO_REMOVE"
  })

# Remove multiple tags
{:ok, _updated} = Client.collection(pb, "posts")
  |> Bosbase.RecordService.update("POST_ID", %{
    "tags-" => ["TAG1", "TAG2"]
  })

Complete Example

# Get existing post
{:ok, post} = Client.collection(pb, "posts")
  |> Bosbase.RecordService.get_one("POST_ID")

IO.inspect(post["tags"])  # ['tag1', 'tag2']

# Remove one tag, add two new ones
{:ok, _updated} = Client.collection(pb, "posts")
  |> Bosbase.RecordService.update("POST_ID", %{
    "tags-" => "tag1",           # Remove
    "tags+" => ["tag3", "tag4"]  # Append
  })

{:ok, updated} = Client.collection(pb, "posts")
  |> Bosbase.RecordService.get_one("POST_ID")

IO.inspect(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
{:ok, comment} = Client.collection(pb, "comments")
  |> Bosbase.RecordService.get_one("COMMENT_ID", %{
    "expand" => "user"
  })

IO.puts(comment["expand"]["user"]["name"])  # "John Doe"
IO.puts(comment["user"])                    # Still the ID: "USER_ID"

Expand Multiple Relations

# Expand multiple relations (comma-separated)
{:ok, comment} = Client.collection(pb, "comments")
  |> Bosbase.RecordService.get_one("COMMENT_ID", %{
    "expand" => "user,post"
  })

IO.puts(comment["expand"]["user"]["name"])   # "John Doe"
IO.puts(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
{:ok, comment} = Client.collection(pb, "comments")
  |> Bosbase.RecordService.get_one("COMMENT_ID", %{
    "expand" => "user,post.tags"
  })

# Access nested expands
IO.inspect(comment["expand"]["post"]["expand"]["tags"])
# Array of tag records

# Expand even deeper
{:ok, post} = Client.collection(pb, "posts")
  |> Bosbase.RecordService.get_one("POST_ID", %{
    "expand" => "user,comments.user"
  })

# Access: post["expand"]["comments"][0]["expand"]["user"]

Expand with List Requests

# List comments with expanded users
{:ok, result} = Client.collection(pb, "comments")
  |> Bosbase.RecordService.get_list(1, 20, %{
    "expand" => "user"
  })

Enum.each(result["items"], fn comment ->
  IO.puts(comment["message"])
  IO.puts(comment["expand"]["user"]["name"])
end)

Expand Single vs Multiple Relations

# Single relation - expand.user is a map
{:ok, post} = Client.collection(pb, "posts")
  |> Bosbase.RecordService.get_one("POST_ID", %{
    "expand" => "user"
  })

IO.inspect(is_map(post["expand"]["user"]))  # true

# Multiple relation - expand.tags is a list
{:ok, post_with_tags} = Client.collection(pb, "posts")
  |> Bosbase.RecordService.get_one("POST_ID", %{
    "expand" => "tags"
  })

IO.inspect(is_list(post_with_tags["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 nil
{:ok, comment} = Client.collection(pb, "comments")
  |> Bosbase.RecordService.get_one("COMMENT_ID", %{
    "expand" => "user"
  })

if comment["expand"]["user"] do
  IO.puts(comment["expand"]["user"]["name"])
else
  IO.puts("User not accessible or not found")
end

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
{:ok, post} = Client.collection(pb, "posts")
  |> Bosbase.RecordService.get_one("POST_ID", %{
    "expand" => "comments_via_post"
  })

# comments_via_post is always a list (even if original field is single)
IO.inspect(post["expand"]["comments_via_post"])
# List of comment records

Back-Relation with Nested Expand

# Get post with comments, and expand each comment's user
{:ok, post} = Client.collection(pb, "posts")
  |> Bosbase.RecordService.get_one("POST_ID", %{
    "expand" => "comments_via_post.user"
  })

# Access nested expands
Enum.each(post["expand"]["comments_via_post"], fn comment ->
  IO.puts(comment["message"])
  IO.puts(comment["expand"]["user"]["name"])
end)

Filtering with Back-Relations

# List posts that have at least one comment containing "hello"
{:ok, result} = Client.collection(pb, "posts")
  |> Bosbase.RecordService.get_list(1, 20, %{
    "filter" => ~s(comments_via_post.message ?~ "hello"),
    "expand" => "comments_via_post.user"
  })

Enum.each(result["items"], fn post ->
  IO.puts(post["title"])
  Enum.each(post["expand"]["comments_via_post"], fn comment ->
    IO.puts("  - #{comment["message"]} by #{comment["expand"]["user"]["name"]}")
  end)
end)

Back-Relation Caveats

Always Multiple: Back-relations are always treated as lists, 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 a list
{:ok, post} = Client.collection(pb, "posts")
  |> Bosbase.RecordService.get_one("POST_ID", %{
    "expand" => "comments_via_post"
  })

# Always a list
IO.inspect(is_list(post["expand"]["comments_via_post"]))  # true

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)
{:ok, post} = Client.collection(pb, "posts")
  |> Bosbase.RecordService.get_one("POST_ID")

# Fetch comments separately with pagination
{:ok, comments} = Client.collection(pb, "comments")
  |> Bosbase.RecordService.get_list(1, 100, %{
    "filter" => ~s(post = "#{post["id"]}"),
    "expand" => "user",
    "sort" => "-created"
  })

Complete Examples

Example 1: Blog Post with Author and Tags

# Create a blog post with relations
{:ok, post} = Client.collection(pb, "posts")
  |> Bosbase.RecordService.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
{:ok, full_post} = Client.collection(pb, "posts")
  |> Bosbase.RecordService.get_one(post["id"], %{
    "expand" => "author,tags"
  })

IO.puts(full_post["title"])
IO.puts("Author: #{full_post["expand"]["author"]["name"]}")
IO.puts("Tags:")
Enum.each(full_post["expand"]["tags"], fn tag ->
  IO.puts("  - #{tag["name"]}")
end)

Example 2: Comment System with Nested Relations

# Create a comment on a post
{:ok, comment} = Client.collection(pb, "comments")
  |> Bosbase.RecordService.create(%{
    "message" => "Great article!",
    "post" => "POST_ID",
    "user" => "USER_ID"
  })

# Get post with all comments and their authors
{:ok, post} = Client.collection(pb, "posts")
  |> Bosbase.RecordService.get_one("POST_ID", %{
    "expand" => "author,comments_via_post.user"
  })

IO.puts("Post: #{post["title"]}")
IO.puts("Author: #{post["expand"]["author"]["name"]}")
IO.puts("Comments (#{length(post["expand"]["comments_via_post"])}):")
Enum.each(post["expand"]["comments_via_post"], fn comment ->
  IO.puts("  #{comment["expand"]["user"]["name"]}: #{comment["message"]}")
end)

Example 3: Dynamic Tag Management

defmodule PostManager do
  def add_tag(pb, post_id, tag_id) do
    Client.collection(pb, "posts")
      |> Bosbase.RecordService.update(post_id, %{
        "tags+" => tag_id
      })
  end

  def remove_tag(pb, post_id, tag_id) do
    Client.collection(pb, "posts")
      |> Bosbase.RecordService.update(post_id, %{
        "tags-" => tag_id
      })
  end

  def get_post_with_tags(pb, post_id) do
    Client.collection(pb, "posts")
      |> Bosbase.RecordService.get_one(post_id, %{
        "expand" => "tags"
      })
  end
end

# Usage
PostManager.add_tag(pb, "POST_ID", "NEW_TAG_ID")
{:ok, post} = PostManager.get_post_with_tags(pb, "POST_ID")

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"] do
  IO.puts(record["expand"]["user"]["name"])
end

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