I am creating the following tables in Rails (with Postgresql):

• User: a user can be in one or more groups and can be the owner of one or more groups
• Group: a group has an admin (which is one of the users) and one or more users
• Participant: a participant the table in between the other two to connect them

The models are like this:

#User.rb

class User < ApplicationRecord
  has_many :groups
  has_many :participants
end

#Group.rb

class Group < ApplicationRecord
  belongs_to :user
  has_many :participants
end

#Participant.rb

class Participant < ApplicationRecord
  belongs_to :group
  belongs_to :user
end

After this, I did some migration to change the name of the user_id that the Group would have, into admin_id:

class ChangeForeignKeyForGroups < ActiveRecord::Migration[6.1]
  def change
    rename_column :groups, :user_id, :admin_id
  end
end

So now, my schema looks like this:

  create_table "groups", force: :cascade do |t|
    t.string "name"
    t.text "description"
    t.integer "participants"
    t.bigint "admin_id", null: false
    t.datetime "created_at", precision: 6, null: false
    t.datetime "updated_at", precision: 6, null: false
    t.index ["admin_id"], name: "index_groups_on_admin_id"
  end

  create_table "participants", force: :cascade do |t|
    t.bigint "group_id", null: false
    t.bigint "user_id", null: false
    t.datetime "created_at", precision: 6, null: false
    t.datetime "updated_at", precision: 6, null: false
    t.index ["group_id"], name: "index_participants_on_group_id"
    t.index ["user_id"], name: "index_participants_on_user_id"
  end

  create_table "users", force: :cascade do |t|
    t.string "email", default: "", null: false
    t.string "encrypted_password", default: "", null: false
    t.string "reset_password_token"
    t.datetime "reset_password_sent_at"
    t.datetime "remember_created_at"
    t.datetime "created_at", precision: 6, null: false
    t.datetime "updated_at", precision: 6, null: false
    t.string "username"
    t.index ["email"], name: "index_users_on_email", unique: true
    t.index ["reset_password_token"], name: "index_users_on_reset_password_token", unique: true

The problem is that once I try to create a Group in seeds, I keep having this error:

pry(main)> group = Group.new(name: "Test Group", description: "blablabal")
=> #<Group:0x00007fdbcdd03f80 id: nil, name: "Test Group", description: "blablabal", participants: nil, admin_id: nil, created_at: nil, updated_at: nil>
[5] pry(main)> group.valid?
=> false
[6] pry(main)> group.errors
=> #<ActiveModel::Errors:0x00007fdbcdc7ad98
 @base=#<Group:0x00007fdbcdd03f80 id: nil, name: "Test Group", description: "blablabal", participants: nil, admin_id: nil, created_at: nil, updated_at: nil>,
 @errors=[#<ActiveModel::Error attribute=user, type=blank, options={:message=>:required}>]>

It seems I am missing the user, so I try adding it but it throws the same error:

user = User.first
  User Load (1.4ms)  SELECT "users".* FROM "users" ORDER BY "users"."id" ASC LIMIT $1  [["LIMIT", 1]]
=> #<User id: 7, email: "[email protected]", created_at: "2022-05-31 10:29:06.208673000 +0000", updated_at: "2022-05-31 10:29:06.208673000 +0000", username: "Jose">

pry(main)> group = Group.new(name: "Test Group", description: "blablabal", admin_id: user.id)
=> #<Group:0x00007fdbdd205ad8 id: nil, name: "Test Group", description: "blablabal", participants: nil, admin_id: 7, created_at: nil, updated_at: nil>
[13] pry(main)> group.valid?
=> false
[14] pry(main)> group.errors
=> #<ActiveModel::Errors:0x00007fdbdd1d4578
 @base=#<Group:0x00007fdbdd205ad8 id: nil, name: "Test Group", description: "blablabal", participants: nil, admin_id: 7, created_at: nil, updated_at: nil>,
 @errors=[#<ActiveModel::Error attribute=user, type=blank, options={:message=>:required}>]>

Am I doing something wrong? Have I structure it wrongly? Thanks!