### Connect to SQL Database and Define Relations with ROM

Source: https://rom-rb.org/guides/sql-quick-start/connecting-to-existing-database

This snippet demonstrates how to establish a connection to a PostgreSQL database using ROM and configure a relation for the 'users' table. It infers the schema automatically and enables struct auto-loading. It also shows how to create a new user record and retrieve it.

```ruby
require "rom"

rom = ROM.container(:sql, 'postgres://localhost/my_db', username: 'user', password: 'secret') do |config|
  config.relation(:users) do
    schema(infer: true)
    auto_struct true
  end
end

users = rom.relations[:users]

users.changeset(:create, name: "Jane").commit

jane = users.where(name: "Jane").one

```

--------------------------------

### ROM Container Setup with Custom Commands (Ruby)

Source: https://rom-rb.org/guides/adapters/how-to-build-an-adapter

Illustrates setting up a ROM container with custom relation and command classes using the Array adapter. It shows how to define relation-specific methods, register commands, and then use the container to create, update, and delete data, including using relation methods for filtering before command execution.

```ruby
configuration = ROM::Configuration.new(:array)

class Users < ROM::Relation[:array]
  def by_name(name)
    select { |user| user[:name] == name }
  end
end

class CreateUser < ROM::Commands::Create[:array]
  relation :users
  register_as :create
end

class UpdateUser < ROM::Commands::Update[:array]
  relation :users
  result :one
  register_as :update
end

class DeleteUser < ROM::Commands::Delete[:array]
  relation :users
  result :one
  register_as :delete
end

configuration.register_relation(Users)

configuration.register_command(CreateUser)
configuration.register_command(UpdateUser)
configuration.register_command(DeleteUser)

rom = ROM.container(configuration)

create_users = rom.commands[:users][:create]
update_user = rom.commands[:users][:update]
delete_user = rom.commands[:users][:delete]

create_users.call([{ name: 'Jane' }, { name: 'John' }])

puts rom.relations[:users].to_a.inspect
# [{:name=>"Jane"}, {:name=>"John"}]


puts rom.relations[:users].by_name('Jane').to_a.inspect
# [{:name=>"Jane"}]


update_user.by_name('Jane').call(name: 'Jane Doe')

puts rom.relations[:users].to_a.inspect
# [{:name=>"Jane Doe"}, {:name=>"John"}]


delete_user.by_name('John').call

puts rom.relations[:users].to_a.inspect
# [{:name=>"Jane Doe"}]
```

--------------------------------

### Implement ROM Delete Command (Ruby)

Source: https://rom-rb.org/guides/adapters/how-to-build-an-adapter

Shows how to implement a custom `Delete` command for the Array adapter. It inherits from `ROM::Commands::Delete` and overrides the `execute` method to remove tuples from the source relation. The example illustrates creating a delete command instance and calling it.

```ruby
require 'rom/commands/delete'

module ROM
  module ArrayAdapter
    module Commands
      class Delete < ROM::Commands::Delete
        adapter :array

        def execute
          relation.each { |tuple| source.delete(tuple) }
        end
      end
    end
  end
end

delete_users = ROM::ArrayAdapter::Commands::Delete.new(users)

delete_users.call

puts users.to_a.inspect
# []
```

--------------------------------

### Implement ROM Update Command (Ruby)

Source: https://rom-rb.org/guides/adapters/how-to-build-an-adapter

Details the creation of a custom `Update` command for the Array adapter, inheriting from `ROM::Commands::Update`. The `execute` method is defined to update tuples in the relation with provided attributes. The example demonstrates instantiating and calling the update command.

```ruby
require 'rom/commands/update'

module ROM
  module ArrayAdapter
    module Commands
      class Update < ROM::Commands::Update
        adapter :array

        def execute(attributes)
          relation.each { |tuple| tuple.update(attributes) }
        end
      end
    end
  end
end

update_users = ROM::ArrayAdapter::Commands::Update.new(users)

update_users.call(age: 21)

puts users.to_a.inspect
# [{:name=>"Jane", :age=>21}]
```

--------------------------------

### Implement ROM Create Command (Ruby)

Source: https://rom-rb.org/guides/adapters/how-to-build-an-adapter

Demonstrates how to create a custom `Create` command for the Array adapter in ROM. This involves inheriting from `ROM::Commands::Create` and defining the `execute` method to add tuples to a relation. It shows basic usage with a relation and command object.

```ruby
require 'rom/commands/create'

module ROM
  module ArrayAdapter
    module Commands
      class Create < ROM::Commands::Create
        adapter :array

        def execute(tuples)
          tuples.each { |tuple| relation << tuple }
        end
      end
    end
  end
end

users = ROM::ArrayAdapter::Relation.new(gateway.dataset(:users))
create_users = ROM::ArrayAdapter::Commands::Create.new(users)

create_users.call([{ name: 'Jane' }])

puts users.to_a.inspect
# [{:name=>"Jane"}]
```

--------------------------------

### ROM Array Adapter Gateway Implementation

Source: https://rom-rb.org/guides/adapters/how-to-build-an-adapter

Implements the ROM::Gateway interface for a simple array-based data source. It manages datasets (arrays) and allows retrieving them by name. This is the foundation for accessing data within the adapter.

```ruby
require 'rom'

module ROM
  module ArrayAdapter
    class Gateway < ROM::Gateway
      attr_reader :datasets

      def initialize
        @datasets = Hash.new { |h, k| h[k] = [] }
      end

      def dataset(name)
        datasets[name]
      end

      def dataset?(name)
        datasets.key?(name)
      end
    end
  end
end

gateway = ROM::ArrayAdapter::Gateway.new

users = gateway.dataset(:users)
tasks = gateway.dataset(:tasks)

gateway.dataset?(:users) # true
gateway.dataset?(:tasks) # true

```

--------------------------------

### Registering the ROM Array Adapter

Source: https://rom-rb.org/guides/adapters/how-to-build-an-adapter

Registers the custom array adapter with ROM using `ROM.register_adapter`. This allows ROM to recognize and utilize the adapter for setting up components. It also demonstrates configuring ROM with the registered adapter and defining a custom relation.

```ruby
ROM.register_adapter(:array, ROM::ArrayAdapter)

configuration= ROM::Configuration.new(:array)

class Users < ROM::Relation[:array]
  def by_name(name)
    select { |user| user[:name] == name }
  end
end

configuration.register_relation(Users)

rom = ROM.container(configuration)

users = rom.gateways[:default].dataset(:users)

users << { name: 'Jane' }
users << { name: 'John' }

rom.relations[:users].by_name('Jane').to_a
# [{:name=>"Jane"}]

```

--------------------------------

### ROM Array Adapter Relation Implementation

Source: https://rom-rb.org/guides/adapters/how-to-build-an-adapter

Defines a ROM::Relation subclass for the array adapter. It configures the adapter identifier and uses the `forward` macro to expose array methods like `select` and `reject` directly on the relation. This allows for adapter-specific query capabilities.

```ruby
module ROM
  module ArrayAdapter
    class Relation < ROM::Relation
      # we must configure adapter identifier here
      adapter :array

      forward :select, :reject
    end
  end
end

users = gateway.dataset(:users)

users << { name: 'Jane' }
users << { name: 'John' }

relation = ROM::ArrayAdapter::Relation.new(gateway.dataset(:users))

relation.select { |tuple| tuple[:name] == 'Jane' }.inspect
# #<ROM::ArrayAdapter::Relation dataset=[{:name=>"Jane"}]>

```

--------------------------------

### ROM Array Adapter Relation with Writing Methods

Source: https://rom-rb.org/guides/adapters/how-to-build-an-adapter

Extends the ROM::ArrayAdapter::Relation to include writing capabilities by forwarding methods like `<<` (append) and `delete` to the underlying dataset. It also adds a `count` method for determining the number of records.

```ruby
module ROM
  module ArrayAdapter
    class Relation < ROM::Relation
      adapter :array

      # reading
      forward :select, :reject

      # writing
      forward :<<, :delete

      def count
        dataset.size
      end
    end
  end
end

```

--------------------------------

### Define User and Team Relations in ROM

Source: https://rom-rb.org/guides/sql-how-to/order-by-nested-association-attribute

Defines the ROM relations for 'users' and 'teams' tables, including associations and attributes. Assumes 'users' table has 'created_at' and 'team_id', and 'teams' table has a 'name'.

```ruby
module Relations
  class Users < ROM::Relation[:sql]
    attribute :created_at, ROM::Types::DateTime

    schema(:users) do
      associations do
        belongs_to :team
      end
    end
  end

  class Teams < ROM::Relation[:sql]
    schema(:teams) do
      attribute :name, ROM::Types::String

      associations do
        has_many :users
      end
    end
  end
end
```

--------------------------------

### Order Users by Team Name and Creation Date

Source: https://rom-rb.org/guides/sql-how-to/order-by-nested-association-attribute

Retrieves all users, ordered first by their associated team's name and then by their creation date. This query uses a JOIN to access the 'teams' table and applies multi-column ordering.

```sql
users.join(:teams).order(teams[:name], :created_at)
```

```ruby
users.combine(:teams).join(:teams).order(teams[:name], :created_at).to_a
```

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