Add ProgrammingGroup & ProgrammingGroupMembership

* User can create programming group with other users for exercise
* Submission is shared in a group
* Also adjust specs

app/models/programming_group.rb

@@ -0,0 +1,82 @@
+# frozen_string_literal: true
+
+class ProgrammingGroup < ApplicationRecord
+  include Contributor
+
+  has_many :programming_group_memberships, dependent: :destroy
+  has_many :external_users, through: :programming_group_memberships, source_type: 'ExternalUser', source: :user
+  has_many :internal_users, through: :programming_group_memberships, source_type: 'InternalUser', source: :user
+  belongs_to :exercise
+
+  validate :group_size
+  validate :no_erroneous_users
+  accepts_nested_attributes_for :programming_group_memberships
+
+  def initialize(attributes = nil)
+    @erroneous_users = []
+    super
+  end
+
+  def external_user?
+    false
+  end
+
+  def internal_user?
+    false
+  end
+
+  def self.nested_resource?
+    true
+  end
+
+  def programming_group?
+    true
+  end
+
+  def add(user)
+    # Accessing the `users` method here will preload all users, which is otherwise done during validation.
+    internal_users << user if user.internal_user? && users.exclude?(user)
+    external_users << user if user.external_user? && users.exclude?(user)
+    user
+  end
+
+  def to_s
+    displayname
+  end
+
+  def displayname
+    "Programming Group #{id}"
+  end
+
+  def programming_partner_ids
+    users.map(&:id_with_type)
+  end
+
+  def users
+    internal_users + external_users
+  end
+
+  def users=(users)
+    self.internal_users = []
+    self.external_users = []
+    users.each do |user|
+      next @erroneous_users << user unless user.is_a?(User)
+
+      add(user)
+    end
+  end
+
+  private
+
+  def group_size
+    if users.size < 2
+      errors.add(:base, :size_too_small)
+    end
+  end
+
+  def no_erroneous_users
+    @erroneous_users.each do |partner_id|
+      errors.add(:base, :invalid_partner_id, partner_id:)
+    end
+  end
+end