The cyclic dependency in this test generates a unique constraint violation due to how aliases are assigned to track which entries have already been inserted to the database.

Gemfile

source 'https://rubygems.org'
gem 'sequel'
gem 'fixture_dependencies'
gem 'sqlite3'

groups.yml

all_users:
    id: 1
    name: All Users
    users:
    - john

test.rb

# frozen_string_literal: true

require 'sequel'
require 'fixture_dependencies'

$dir = File.dirname(File.expand_path(__FILE__))

FixtureDependencies.fixture_path = $dir
DB = Sequel.sqlite(File.join($dir, 'classmap_test.sqlite3'))

DB.create_table? :groups do
    primary_key :id
    text :name
end
DB.create_table? :users do
    primary_key :id
    foreign_key :group_id, :groups
    text :name
end

module ClassMapTest
    class User < Sequel::Model(:users)
        many_to_one :group
    end
    FixtureDependencies.class_map[:user] = User

    class Group < Sequel::Model(:groups)
        one_to_many :users
    end
    FixtureDependencies.class_map[:group] = Group
end

begin
    john = FixtureDependencies.load(:user__john)
    p john
ensure
    DB.drop_table :users
    DB.drop_table :groups
end

users.yml

john:
    id: 1
    group: all_users
    name: John Smith

When load(:user__john) is executed, the resulting model is cached as :user__john. Then :class_map_test/group__all_users is created, which references :class_map_test/user__john -- since the name is different, the cached model is not found and it attempts to insert the record. The insert fails because it has already been inserted.