Phoenix Generator

This skill helps generate Phoenix resources using built-in generators with proper patterns and best practices.

When to Use

• Creating new Phoenix contexts
• Adding schemas to existing contexts
• Generating LiveView CRUD interfaces
• Creating JSON/HTML resources
• Scaffolding authentication

Available Generators

Context and Schema Generation

mix phx.gen.context - Generate a context with schema and migration

mix phx.gen.context Accounts User users name:string email:string:unique age:integer

Generates:

• Context module: lib/my_app/accounts.ex
• Schema: lib/my_app/accounts/user.ex
• Migration: priv/repo/migrations/*_create_users.exs
• Test files

mix phx.gen.schema - Schema and migration only (no context)

mix phx.gen.schema Accounts.User users name:string email:string:unique

LiveView Generators

mix phx.gen.live - Full LiveView CRUD with context

mix phx.gen.live Catalog Product products \
  name:string \
  description:text \
  price:decimal \
  sku:string:unique \
  in_stock:boolean

Generates:

• Context and schema
• LiveView modules (Index, Show, Form Component)
• Routes
• Tests

mix phx.gen.live.component - Standalone LiveView component

mix phx.gen.live.component Components.Modal

Traditional Web Generators

mix phx.gen.html - HTML resource with controllers

mix phx.gen.html Blog Post posts title:string body:text published:boolean

mix phx.gen.json - JSON API resource

mix phx.gen.json Shop Product products name:string price:decimal

Field Types and Modifiers

Common Field Types

• string - varchar(255)
• text - unlimited text
• integer - whole numbers
• decimal - precise decimals (for money)
• float - floating point
• boolean - true/false
• date - date only
• time - time only
• datetime - timestamp
• uuid - UUID
• binary - binary data

Field Modifiers

• :unique - adds unique index
• :redact - marks for redaction in logs
• name:string:unique:redact - chain modifiers

References (Associations)

# belongs_to
mix phx.gen.context Blog Post posts \
  title:string \
  body:text \
  user_id:references:users

# Specify custom reference
mix phx.gen.context Comments Comment comments \
  body:text \
  post_id:references:posts \
  author_id:references:users

Best Practices

1. Context Naming

• Use plural for context: Accounts, Catalog, Blog
• Use singular for schema: User, Product, Post
• Table name matches schema plural: users, products, posts

2. Field Selection

# Good: Specific types
price:decimal         # For money
published_at:datetime # For timestamps
active:boolean        # For flags

# Avoid: Wrong types
price:float          # Loses precision for money
published:string     # Use datetime or boolean

3. Associations

# Always specify references explicitly
user_id:references:users

# Don't use bare integer for foreign keys
user_id:integer  # Missing association metadata

4. Decimal Precision

For money fields, update migration:

# Generated
add :price, :decimal

# Best Practice - specify precision
add :price, :decimal, precision: 10, scale: 2

5. String Length Limits

Update schema for validation:

# Add to changeset
|> validate_length(:name, max: 100)
|> validate_length(:email, max: 160)

Generator Workflow

Step 1: Plan Schema

# List fields with types
name:string
email:string:unique
age:integer
bio:text
admin:boolean
inserted_at:datetime
updated_at:datetime  # automatic with timestamps()

Step 2: Generate Resource

# Choose appropriate generator
mix phx.gen.live Accounts User users \
  name:string \
  email:string:unique \
  age:integer \
  bio:text \
  admin:boolean

Step 3: Review Generated Files

• Check migration for proper indexes and constraints
• Review schema for additional validations needed
• Update context functions as needed
• Review tests and add edge cases

Step 4: Customize Migration

# Before running migration, edit it
vim priv/repo/migrations/*_create_users.exs

Add:

• Check constraints
• Default values
• Additional indexes
• Precision for decimals

Step 5: Add Routes (if needed)

# lib/my_app_web/router.ex
scope "/", MyAppWeb do
  pipe_through :browser

  live "/users", UserLive.Index, :index
  live "/users/new", UserLive.Index, :new
  live "/users/:id/edit", UserLive.Index, :edit
  live "/users/:id", UserLive.Show, :show
  live "/users/:id/show/edit", UserLive.Show, :edit
end

Step 6: Run Migration

mix ecto.migrate

Step 7: Update Tests

• Add validation tests
• Add association tests
• Test edge cases

Common Patterns

Money Fields

mix phx.gen.live Shop Product products \
  name:string \
  price:decimal \
  sale_price:decimal

Then update migration:

add :price, :decimal, precision: 10, scale: 2, null: false
add :sale_price, :decimal, precision: 10, scale: 2

Soft Deletes

Add to schema:

deleted_at:datetime

Update migration:

add :deleted_at, :utc_datetime
create index(:products, [:deleted_at])

Polymorphic Associations

mix phx.gen.schema Comments.Comment comments \
  body:text \
  commentable_type:string \
  commentable_id:integer

Multi-tenant (Tenant ID)

mix phx.gen.live Accounts User users \
  organization_id:references:organizations \
  name:string \
  email:string

Add unique constraint on tenant + field:

create unique_index(:users, [:organization_id, :email])

Undoing Generators

If you need to rollback:

# Rollback migration
mix ecto.rollback

# Delete generated files manually or use:
mix phx.gen.context --undo Accounts User users

Troubleshooting

Module already exists:

• Generator won't overwrite existing files
• Use --merge-with-existing-context to add to existing context
• Or manually integrate generated code

Routes not working:

• Check router.ex was updated
• Verify pipe_through matches (:browser or :api)
• Restart server after adding routes

Migration fails:

• Check for duplicate table names
• Verify referenced tables exist
• Ensure field types are valid

Generator Flags

# Skip migration
mix phx.gen.context Accounts User users name:string --no-migration

# Binary ID (UUID)
mix phx.gen.schema Accounts User users name:string --binary-id

# Merge with existing context
mix phx.gen.context Accounts Profile profiles bio:text --merge-with-existing-context

# Specify table name
mix phx.gen.schema Accounts User my_users name:string --table my_users