For example, we have this Channel class:

class Channel
  attr_reader :id, :name

  def initialize(id, name) 
    @id = id
    @name = name
  end
end

1.

What we have:

One list of objects:

current_channels = [Channel.new(1, 'channel1'), Channel.new(2, 'channel2'), Channel.new(3, 'channel3')]

1.1 Bad solution

current_channel_ids = []

current_channels.each do |channel|
  current_channel_ids << channel.id
end

current_channel_ids = current_channels.map { |channel| channel.id }

current_channel_ids = current_channels.map(&:id)
 

2.

What we have:

Two lists of objects:

current_channels = [Channel.new(1, 'channel1'), Channel.new(2, 'channel2'), Channel.new(3, 'channel3')]
need_to_be_assigned_channels = [ Channel.new(1, 'channel1'), Channel.new(4, 'channel4')]

Solution:

Before each solution, get the an array of ids from current_channels:

current_channel_ids = current_channels.map(&:id)

2.1 Bad solution

need_to_be_assigned_channels.each do |channel|
  unless current_channel_ids.include?(channel.id)
    current_channels << channel
  end
end

need_to_be_assigned_channels.each do |channel|
  next if current_channel_ids.include?(channel.id)
  current_channels << channel
end

current_channels.concat(need_to_be_assigned_channels.reject { |assigned_channel| current_channel_ids.include?(assigned_channel.id) }

What we have:

Two lists of objects:

current_channels = [Channel.new(1, 'channel1'), Channel.new(2, 'channel2'), Channel.new(3, 'channel3')]
need_to_be_deleted_channels = [ Channel.new(1, 'channel1'), Channel.new(3, 'channel3')]

Solution:

Before each solution, get the an array of ids from current_channels:

current_channel_ids = current_channels.map(&:id)

current_channels.each do |channel|
  next unless need_to_be_deleted_channel_ids.include?(channel.id)
  current_channels.delete(channel)
end

current_channels.delete_if { |channel| need_to_be_deleted_channel_ids.include?(channel.id) }