Active Record allows you to define relationships in 2 directions. But unfortunately, it is not always smart enough to detect when a relationship on one side is the same as the opposite direction. This aims to help you discover where and why rails is not helping you out.

reasons.rb

class Reason
  def reason_inverse_name(src, ref)
    # def inverse_name; delegate_reflection.send(:inverse_name); end
    ref = ref.respond_to?(:delegate_reflection) ? ref.delegate_reflection : ref

    if ref.options[:inverse_of]
      # will this false positive a :through?
      if ref.options[:inverse_of] == ref.send(:automatic_inverse_of)
        "UNNEEDED: `options[:inverse_of]`"
      else
        "`options[:inverse_of]`"
      end
    else
      reason_automatic_inverse_of(src, ref)
    end
  end

  def reason_automatic_inverse_of(src, ref)
    reason = reason_can_find_inverse_of_automatically?(src, ref, ref)
    return reason if reason
    inverse_name = ActiveSupport::Inflector.underscore(ref.options[:as] || ref.active_record.name.demodulize).to_sym

    begin
      reflection = ref.klass._reflect_on_association(inverse_name)
    rescue NameError
      # Give up: we couldn't compute the klass type so we won't be able
      # to find any associations either.
      return "cant compute #{ref.klass}.#{inverse_name}.klass"
    end

    reason_valid_inverse_reflection?(src, ref, reflection, "#{reflection.try(:active_record).try(:name) || ref.klass}.#{inverse_name}") 
  end

  # return reason it is BAD
  def reason_can_find_inverse_of_automatically?(src, ref, reflection) # inverse ( return value if failing)
    case
    when reflection.options[:inverse_of] == false
      "detection disabled with `:inverse_of => false`"
    when !ref.class::VALID_AUTOMATIC_INVERSE_MACROS.include?(reflection.macro)
      "`#{ref.macro}` not supported"
    when !(opts=ref.class::INVALID_AUTOMATIC_INVERSE_OPTIONS.select { |opt| reflection.options[opt] }).empty?
      "option `:#{opts.join("`, `:")}` not supported"
    when reflection.scope
      # do default scopes even come into play?
      "`scope` not supported (#{reflection.klass})#{" (there is a `default_scope`)" if reflection.klass.default_scopes.size > 0}"
    end
  end

  def reason_valid_inverse_reflection?(src, ref, reflection, name) # inverse (return value if valid or not)
    case
    when !reflection
      # would be nice to get all the STI ancestors
      options = [ref.klass, ref.klass.base_class].uniq.flat_map do |k|
        k.reflections.values.select { |r|
          (r.klass.base_class rescue nil) == src.base_class
        }.map(&:name)
      end
      possible = options.empty? ? "" : " (suggested values: `#{options.join("`, `")})`" 
      "no reverse association `#{name}`#{possible}"
    when ref.klass.name != reflection.active_record.name
      "`#{name}` exists, but wrong parent class (want `#{ref.klass.name}`)"
    else
      reason_can_find_inverse_of_automatically?(src, ref, reflection) # return value if valid or not
    end
  end

  def reasons(*srcs)
    srcs = [Tenant, Vm, VmOrTemplate, Host, EmsCluster] if srcs.empty?

    puts "|reference|reverse|reason"
    puts "|---------|-----|------"
    srcs.each do |src|
      src.reflections.values.each_with_index do |ref, i|
        value = ref.send(:inverse_name)
        why = reason_inverse_name(src, ref)
        puts "|`#{src.name}.#{ref.name}`|#{value ? "`#{ref.klass.name}.#{value}`" : "**NO**"}|#{why || "automatic"}|"
      end
    end
    nil
  end
end

Reason.new.reasons(Tenant, Vm, VmOrTemplate, Host, EmsCluster)

results.md

reference	reverse	reason
Tenant.taggings	NO	option :polymorphic not supported
Tenant.tags	NO	option :through not supported
Tenant.providers	Provider.tenant	automatic
Tenant.ext_management_systems	ExtManagementSystem.tenant	automatic
Tenant.vm_or_templates	VmOrTemplate.tenant	automatic
Tenant.vms	NO	VmOrTemplate.tenant exists, but wrong parent class (want Vm)
Tenant.miq_templates	NO	VmOrTemplate.tenant exists, but wrong parent class (want MiqTemplate)
Tenant.service_template_catalogs	ServiceTemplateCatalog.tenant	automatic
Tenant.service_templates	ServiceTemplate.tenant	automatic
Tenant.tenant_quotas	TenantQuota.tenant	automatic
Tenant.miq_groups	MiqGroup.tenant	automatic
Tenant.users	NO	option :through not supported
Tenant.ae_domains	MiqAeDomain.tenant	automatic
Tenant.miq_requests	MiqRequest.tenant	automatic
Tenant.miq_request_tasks	MiqRequestTask.tenant	automatic
Tenant.services	Service.tenant	automatic
Tenant.default_miq_group	MiqGroup.tenant	UNNEEDED: options[:inverse_of]
Vm.snapshots	Snapshot.vm_or_template	automatic
Vm.compliances	NO	option :polymorphic not supported
Vm.evm_owner	NO	no reverse association User.vm_or_template (suggested values: vms, miq_templates)
Vm.miq_group	NO	no reverse association MiqGroup.vm_or_template (suggested values: vms, miq_templates)
Vm.custom_attributes	NO	option :polymorphic not supported
Vm.miq_custom_attributes	NO	scope not supported (CustomAttribute)
Vm.ems_custom_attributes	NO	scope not supported (CustomAttribute)
Vm.miq_server	NO	option :foreign_key not supported
Vm.operating_system	OperatingSystem.vm_or_template	automatic
Vm.hardware	Hardware.vm_or_template	automatic
Vm.host	NO	no reverse association Host.vm_or_template (suggested values: vms_and_templates, vms, miq_templates)
Vm.ems_cluster	NO	no reverse association EmsCluster.vm_or_template (suggested values: vms_and_templates, miq_templates, vms)
Vm.storage	NO	no reverse association Storage.vm_or_template (suggested values: vms_and_templates, miq_templates, vms, all_vms_and_templates, all_miq_templates, all_vms)
Vm.repositories	NO	option :through not supported
Vm.storages	NO	has_and_belongs_to_many not supported
Vm.ext_management_system	NO	option :foreign_key not supported
Vm.miq_provision	NO	MiqRequestTask.destination exists, but wrong parent class (want MiqProvision)
Vm.miq_provisions_from_template	NO	MiqRequestTask.source exists, but wrong parent class (want MiqProvision)
Vm.miq_provision_vms	NO	option :through not supported
Vm.miq_provision_requests	NO	MiqRequest.source exists, but wrong parent class (want MiqProvisionRequest)
Vm.guest_applications	GuestApplication.vm_or_template	automatic
Vm.patches	Patch.vm_or_template	automatic
Vm.accounts	Account.vm_or_template	automatic
Vm.users	NO	scope not supported (Account)
Vm.groups	NO	scope not supported (Account)
Vm.system_services	SystemService.vm_or_template	automatic
Vm.win32_services	NO	scope not supported (SystemService)
Vm.kernel_drivers	NO	scope not supported (SystemService)
Vm.filesystem_drivers	NO	scope not supported (SystemService)
Vm.linux_initprocesses	NO	scope not supported (SystemService)
Vm.filesystems	NO	option :polymorphic not supported
Vm.directories	NO	scope not supported (Filesystem)
Vm.files	NO	scope not supported (Filesystem)
Vm.scan_histories	ScanHistory.vm_or_template	automatic
Vm.lifecycle_events	LifecycleEvent.vm_or_template	automatic
Vm.advanced_settings	NO	option :polymorphic not supported
Vm.registry_items	RegistryItem.vm_or_template	automatic
Vm.metrics	NO	option :polymorphic not supported
Vm.metric_rollups	NO	option :polymorphic not supported
Vm.vim_performance_states	NO	option :polymorphic not supported
Vm.storage_files	StorageFile.vm_or_template	automatic
Vm.storage_files_files	NO	scope not supported (StorageFile)
Vm.ems_events	NO	scope not supported (EmsEvent)
Vm.ems_events_src	NO	EventStream.vm_or_template exists, but wrong parent class (want EmsEvent)
Vm.ems_events_dest	NO	option :foreign_key not supported
Vm.policy_events	NO	scope not supported (PolicyEvent)
Vm.miq_events	NO	EventStream.target exists, but wrong parent class (want MiqEvent)
Vm.miq_alert_statuses	NO	option :polymorphic not supported
Vm.miq_cim_instance	NO	option :polymorphic not supported
Vm.service_resources	NO	option :polymorphic not supported
Vm.direct_services	NO	option :through not supported
Vm.tenant	NO	no reverse association Tenant.vm_or_template (suggested values: vm_or_templates, vms, miq_templates)
Vm.taggings	NO	option :polymorphic not supported
Vm.tags	NO	option :through not supported
Vm.all_relationships	NO	option :polymorphic not supported
Vm.drift_states	NO	option :polymorphic not supported
Vm.first_drift_state	NO	scope not supported (DriftState)
Vm.last_drift_state	NO	scope not supported (DriftState)
Vm.first_drift_state_timestamp_rec	NO	scope not supported (DriftState)
Vm.last_drift_state_timestamp_rec	NO	scope not supported (DriftState)
Vm.vim_performance_operating_ranges	NO	option :polymorphic not supported
VmOrTemplate.snapshots	Snapshot.vm_or_template	automatic
VmOrTemplate.compliances	NO	option :polymorphic not supported
VmOrTemplate.evm_owner	NO	no reverse association User.vm_or_template (suggested values: vms, miq_templates)
VmOrTemplate.miq_group	NO	no reverse association MiqGroup.vm_or_template (suggested values: vms, miq_templates)
VmOrTemplate.custom_attributes	NO	option :polymorphic not supported
VmOrTemplate.miq_custom_attributes	NO	scope not supported (CustomAttribute)
VmOrTemplate.ems_custom_attributes	NO	scope not supported (CustomAttribute)
VmOrTemplate.miq_server	NO	option :foreign_key not supported
VmOrTemplate.operating_system	OperatingSystem.vm_or_template	automatic
VmOrTemplate.hardware	Hardware.vm_or_template	automatic
VmOrTemplate.host	NO	no reverse association Host.vm_or_template (suggested values: vms_and_templates, vms, miq_templates)
VmOrTemplate.ems_cluster	NO	no reverse association EmsCluster.vm_or_template (suggested values: vms_and_templates, miq_templates, vms)
VmOrTemplate.storage	NO	no reverse association Storage.vm_or_template (suggested values: vms_and_templates, miq_templates, vms, all_vms_and_templates, all_miq_templates, all_vms)
VmOrTemplate.repositories	NO	option :through not supported
VmOrTemplate.storages	NO	has_and_belongs_to_many not supported
VmOrTemplate.ext_management_system	NO	option :foreign_key not supported
VmOrTemplate.miq_provision	NO	MiqRequestTask.destination exists, but wrong parent class (want MiqProvision)
VmOrTemplate.miq_provisions_from_template	NO	MiqRequestTask.source exists, but wrong parent class (want MiqProvision)
VmOrTemplate.miq_provision_vms	NO	option :through not supported
VmOrTemplate.miq_provision_requests	NO	MiqRequest.source exists, but wrong parent class (want MiqProvisionRequest)
VmOrTemplate.guest_applications	GuestApplication.vm_or_template	automatic
VmOrTemplate.patches	Patch.vm_or_template	automatic
VmOrTemplate.accounts	Account.vm_or_template	automatic
VmOrTemplate.users	NO	scope not supported (Account)
VmOrTemplate.groups	NO	scope not supported (Account)
VmOrTemplate.system_services	SystemService.vm_or_template	automatic
VmOrTemplate.win32_services	NO	scope not supported (SystemService)
VmOrTemplate.kernel_drivers	NO	scope not supported (SystemService)
VmOrTemplate.filesystem_drivers	NO	scope not supported (SystemService)
VmOrTemplate.linux_initprocesses	NO	scope not supported (SystemService)
VmOrTemplate.filesystems	NO	option :polymorphic not supported
VmOrTemplate.directories	NO	scope not supported (Filesystem)
VmOrTemplate.files	NO	scope not supported (Filesystem)
VmOrTemplate.scan_histories	ScanHistory.vm_or_template	automatic
VmOrTemplate.lifecycle_events	LifecycleEvent.vm_or_template	automatic
VmOrTemplate.advanced_settings	NO	option :polymorphic not supported
VmOrTemplate.registry_items	RegistryItem.vm_or_template	automatic
VmOrTemplate.metrics	NO	option :polymorphic not supported
VmOrTemplate.metric_rollups	NO	option :polymorphic not supported
VmOrTemplate.vim_performance_states	NO	option :polymorphic not supported
VmOrTemplate.storage_files	StorageFile.vm_or_template	automatic
VmOrTemplate.storage_files_files	NO	scope not supported (StorageFile)
VmOrTemplate.ems_events	NO	scope not supported (EmsEvent)
VmOrTemplate.ems_events_src	NO	EventStream.vm_or_template exists, but wrong parent class (want EmsEvent)
VmOrTemplate.ems_events_dest	NO	option :foreign_key not supported
VmOrTemplate.policy_events	NO	scope not supported (PolicyEvent)
VmOrTemplate.miq_events	NO	EventStream.target exists, but wrong parent class (want MiqEvent)
VmOrTemplate.miq_alert_statuses	NO	option :polymorphic not supported
VmOrTemplate.miq_cim_instance	NO	option :polymorphic not supported
VmOrTemplate.service_resources	NO	option :polymorphic not supported
VmOrTemplate.direct_services	NO	option :through not supported
VmOrTemplate.tenant	NO	no reverse association Tenant.vm_or_template (suggested values: vm_or_templates, vms, miq_templates)
VmOrTemplate.taggings	NO	option :polymorphic not supported
VmOrTemplate.tags	NO	option :through not supported
VmOrTemplate.all_relationships	NO	option :polymorphic not supported
VmOrTemplate.drift_states	NO	option :polymorphic not supported
VmOrTemplate.first_drift_state	NO	scope not supported (DriftState)
VmOrTemplate.last_drift_state	NO	scope not supported (DriftState)
VmOrTemplate.first_drift_state_timestamp_rec	NO	scope not supported (DriftState)
VmOrTemplate.last_drift_state_timestamp_rec	NO	scope not supported (DriftState)
VmOrTemplate.vim_performance_operating_ranges	NO	option :polymorphic not supported
Host.ext_management_system	NO	option :foreign_key not supported
Host.ems_cluster	NO	no reverse association EmsCluster.host (suggested values: hosts)
Host.operating_system	NO	no reverse association OperatingSystem.host
Host.hardware	Hardware.host	automatic
Host.vms_and_templates	VmOrTemplate.host	automatic
Host.vms	NO	VmOrTemplate.host exists, but wrong parent class (want Vm)
Host.miq_templates	NO	VmOrTemplate.host exists, but wrong parent class (want MiqTemplate)
Host.storages	NO	has_and_belongs_to_many not supported
Host.switches	Switch.host	automatic
Host.patches	Patch.host	automatic
Host.system_services	SystemService.host	automatic
Host.host_services	NO	option :foreign_key not supported
Host.metrics	NO	option :polymorphic not supported
Host.metric_rollups	NO	option :polymorphic not supported
Host.vim_performance_states	NO	option :polymorphic not supported
Host.ems_events	NO	scope not supported (EmsEvent)
Host.ems_events_src	NO	EventStream.host exists, but wrong parent class (want EmsEvent)
Host.ems_events_dest	NO	option :foreign_key not supported
Host.policy_events	NO	scope not supported (PolicyEvent)
Host.guest_applications	GuestApplication.host	automatic
Host.miq_events	NO	EventStream.target exists, but wrong parent class (want MiqEvent)
Host.filesystems	NO	option :polymorphic not supported
Host.directories	NO	scope not supported (Filesystem)
Host.files	NO	scope not supported (Filesystem)
Host.accounts	Account.host	automatic
Host.users	NO	option :foreign_key not supported
Host.groups	NO	option :foreign_key not supported
Host.advanced_settings	NO	option :polymorphic not supported
Host.miq_alert_statuses	NO	option :polymorphic not supported
Host.miq_cim_instance	NO	option :polymorphic not supported
Host.host_service_groups	HostServiceGroup.host	automatic
Host.custom_attributes	NO	option :polymorphic not supported
Host.miq_custom_attributes	NO	scope not supported (CustomAttribute)
Host.ems_custom_attributes	NO	scope not supported (CustomAttribute)
Host.filesystems_custom_attributes	NO	option :through not supported
Host.taggings	NO	option :polymorphic not supported
Host.tags	NO	option :through not supported
Host.all_relationships	NO	option :polymorphic not supported
Host.drift_states	NO	option :polymorphic not supported
Host.first_drift_state	NO	scope not supported (DriftState)
Host.last_drift_state	NO	scope not supported (DriftState)
Host.first_drift_state_timestamp_rec	NO	scope not supported (DriftState)
Host.last_drift_state_timestamp_rec	NO	scope not supported (DriftState)
Host.vim_performance_operating_ranges	NO	option :polymorphic not supported
Host.authentications	NO	option :polymorphic not supported
Host.compliances	NO	option :polymorphic not supported
EmsCluster.taggings	NO	option :polymorphic not supported
EmsCluster.tags	NO	option :through not supported
EmsCluster.ext_management_system	NO	option :foreign_key not supported
EmsCluster.hosts	Host.ems_cluster	automatic
EmsCluster.vms_and_templates	VmOrTemplate.ems_cluster	automatic
EmsCluster.miq_templates	NO	VmOrTemplate.ems_cluster exists, but wrong parent class (want MiqTemplate)
EmsCluster.vms	NO	VmOrTemplate.ems_cluster exists, but wrong parent class (want Vm)
EmsCluster.metrics	NO	option :polymorphic not supported
EmsCluster.metric_rollups	NO	option :polymorphic not supported
EmsCluster.vim_performance_states	NO	option :polymorphic not supported
EmsCluster.policy_events	NO	scope not supported (PolicyEvent)
EmsCluster.miq_events	NO	EventStream.target exists, but wrong parent class (want MiqEvent)
EmsCluster.drift_states	NO	option :polymorphic not supported
EmsCluster.first_drift_state	NO	scope not supported (DriftState)
EmsCluster.last_drift_state	NO	scope not supported (DriftState)
EmsCluster.first_drift_state_timestamp_rec	NO	scope not supported (DriftState)
EmsCluster.last_drift_state_timestamp_rec	NO	scope not supported (DriftState)
EmsCluster.all_relationships	NO	option :polymorphic not supported
EmsCluster.vim_performance_operating_ranges	NO	option :polymorphic not supported
=> true
irb(main):002:0>