diff --git a/addons/mail/static/src/model/model_manager.js b/addons/mail/static/src/model/model_manager.js
index 3c44839ede6..24594acae96 100644
--- a/addons/mail/static/src/model/model_manager.js
+++ b/addons/mail/static/src/model/model_manager.js
@@ -638,6 +638,9 @@ export class ModelManager {
                 ) {
                     throw new Error(`Mismatched relation types: ${field} on ${Model} (${field.relationType}) and ${inverseField} on ${RelatedModel} (${inverseField.relationType}).`);
                 }
+                if (field.required && !inverseField.isCausal) {
+                    throw new Error(`${field} on ${Model} is required but its inverse ${inverseField} on ${RelatedModel} is not causal.`);
+                }
             }
             for (const identifyingField of Model.__identifyingFieldsFlattened) {
                 const field = Model.__fieldMap[identifyingField];
@@ -1070,7 +1073,10 @@ export class ModelManager {
             relFunc(Model.name, { inverse: field.fieldName }),
             {
                 fieldName: `_inverse_${Model}/${field.fieldName}`,
-            }
+                // Note that an identifying field is not necessarily defined as
+                // `required` (example: identifyingFields with an OR). 
+                isCausal: field.required || Model.__identifyingFieldsFlattened.has(field.fieldName),
+            },
         ));
         return inverseField;
     }
diff --git a/addons/mail/static/src/models/attachment/attachment.js b/addons/mail/static/src/models/attachment/attachment.js
index 2c94f255300..16efacb45c9 100644
--- a/addons/mail/static/src/models/attachment/attachment.js
+++ b/addons/mail/static/src/models/attachment/attachment.js
@@ -279,29 +279,12 @@ registerModel({
         activities: many2many('Activity', {
             inverse: 'attachments',
         }),
-        /**
-         * States the attachment cards that are displaying this attachment.
-         */
-        attachmentCards: one2many('AttachmentCard', {
-            inverse: 'attachment',
-            isCausal: true,
-        }),
-        /**
-         * States the attachment images that are displaying this attachment.
-         */
-        attachmentImages: one2many('AttachmentImage', {
-            inverse: 'attachment',
-            isCausal: true,
-        }),
         /**
          * States the attachment lists that are displaying this attachment.
          */
         attachmentLists: many2many('AttachmentList', {
             inverse: 'attachments',
         }),
-        attachmentViewers: many2many('AttachmentViewer', {
-            inverse: 'attachments',
-        }),
         checksum: attr(),
         /**
          * States on which composer this attachment is currently being created.
diff --git a/addons/mail/static/src/models/attachment_card/attachment_card.js b/addons/mail/static/src/models/attachment_card/attachment_card.js
index c71c39f9d84..770e723263e 100644
--- a/addons/mail/static/src/models/attachment_card/attachment_card.js
+++ b/addons/mail/static/src/models/attachment_card/attachment_card.js
@@ -66,7 +66,6 @@ registerModel({
          * Determines the attachment of this card.
          */
         attachment: many2one('Attachment', {
-            inverse: 'attachmentCards',
             readonly: true,
             required: true,
         }),
diff --git a/addons/mail/static/src/models/attachment_image/attachment_image.js b/addons/mail/static/src/models/attachment_image/attachment_image.js
index 8a4c58d3353..9c4c9d57035 100644
--- a/addons/mail/static/src/models/attachment_image/attachment_image.js
+++ b/addons/mail/static/src/models/attachment_image/attachment_image.js
@@ -108,7 +108,6 @@ registerModel({
          * Determines the attachment of this attachment image..
          */
         attachment: many2one('Attachment', {
-            inverse: 'attachmentImages',
             readonly: true,
             required: true,
         }),
diff --git a/addons/mail/static/src/models/attachment_list/attachment_list.js b/addons/mail/static/src/models/attachment_list/attachment_list.js
index 81025830caa..a18b1bb9ce2 100644
--- a/addons/mail/static/src/models/attachment_list/attachment_list.js
+++ b/addons/mail/static/src/models/attachment_list/attachment_list.js
@@ -80,13 +80,6 @@ registerModel({
             inverse: 'attachmentList',
             isCausal: true,
         }),
-        /**
-         * Determines the attachment viewers displaying this attachment list (if any).
-         */
-        attachmentViewers: one2many('AttachmentViewer', {
-            inverse: 'attachmentList',
-            isCausal: true,
-        }),
         /**
          * Link with a chatter to handle attachments.
          */
diff --git a/addons/mail/static/src/models/attachment_viewer/attachment_viewer.js b/addons/mail/static/src/models/attachment_viewer/attachment_viewer.js
index 58a05bc9170..6e485d3b408 100644
--- a/addons/mail/static/src/models/attachment_viewer/attachment_viewer.js
+++ b/addons/mail/static/src/models/attachment_viewer/attachment_viewer.js
@@ -40,12 +40,10 @@ registerModel({
         }),
         attachment: many2one('Attachment'),
         attachmentList: many2one('AttachmentList', {
-            inverse: 'attachmentViewers',
             readonly: true,
             required: true,
         }),
         attachments: many2many('Attachment', {
-            inverse: 'attachmentViewers',
             related: 'attachmentList.viewableAttachments',
         }),
         /**
diff --git a/addons/mail/static/src/models/channel_invitation_form/channel_invitation_form.js b/addons/mail/static/src/models/channel_invitation_form/channel_invitation_form.js
index 6ada1d1b0b7..7bf92edaf0c 100644
--- a/addons/mail/static/src/models/channel_invitation_form/channel_invitation_form.js
+++ b/addons/mail/static/src/models/channel_invitation_form/channel_invitation_form.js
@@ -210,10 +210,6 @@ registerModel({
         inviteButtonText: attr({
             compute: '_computeInviteButtonText',
         }),
-        popoverView: one2one('PopoverView', {
-            inverse: 'channelInvitationForm',
-            isCausal: true,
-        }),
         /**
          * States the OWL ref of the "search" input of this channel invitation
          * form. Useful to be able to focus it.
diff --git a/addons/mail/static/src/models/composer/composer.js b/addons/mail/static/src/models/composer/composer.js
index cdfadd21013..2641e8cc710 100644
--- a/addons/mail/static/src/models/composer/composer.js
+++ b/addons/mail/static/src/models/composer/composer.js
@@ -135,10 +135,6 @@ registerModel({
             compute: '_computeCanPostMessage',
             default: false,
         }),
-        composerViews: one2many('ComposerView', {
-            inverse: 'composer',
-            isCausal: true,
-        }),
         /**
          * This field determines whether some attachments linked to this
          * composer are being uploaded.
diff --git a/addons/mail/static/src/models/composer_view/composer_view.js b/addons/mail/static/src/models/composer_view/composer_view.js
index 1c1f4a88c07..6415b376b6c 100644
--- a/addons/mail/static/src/models/composer_view/composer_view.js
+++ b/addons/mail/static/src/models/composer_view/composer_view.js
@@ -871,7 +871,6 @@ registerModel({
          */
         composer: many2one('Composer', {
             compute: '_computeComposer',
-            inverse: 'composerViews',
             required: true,
         }),
         /**
diff --git a/addons/mail/static/src/models/message/message.js b/addons/mail/static/src/models/message/message.js
index b02cb739c20..8019b901fcf 100644
--- a/addons/mail/static/src/models/message/message.js
+++ b/addons/mail/static/src/models/message/message.js
@@ -701,10 +701,6 @@ registerModel({
             isCausal: true,
         }),
         message_type: attr(),
-        messageSeenIndicators: one2many('MessageSeenIndicator', {
-            inverse: 'message',
-            isCausal: true,
-        }),
         /**
          * States the views that are displaying this message.
          */
diff --git a/addons/mail/static/src/models/message_seen_indicator/message_seen_indicator.js b/addons/mail/static/src/models/message_seen_indicator/message_seen_indicator.js
index 223cb57d52a..1bdd55c680d 100644
--- a/addons/mail/static/src/models/message_seen_indicator/message_seen_indicator.js
+++ b/addons/mail/static/src/models/message_seen_indicator/message_seen_indicator.js
@@ -226,7 +226,6 @@ registerModel({
          * The message concerned by this seen indicator.
          */
         message: many2one('Message', {
-            inverse: 'messageSeenIndicators',
             readonly: true,
             required: true,
         }),
diff --git a/addons/mail/static/src/models/partner/partner.js b/addons/mail/static/src/models/partner/partner.js
index 539a50cd502..3f506898fca 100644
--- a/addons/mail/static/src/models/partner/partner.js
+++ b/addons/mail/static/src/models/partner/partner.js
@@ -446,10 +446,6 @@ registerModel({
         nameOrDisplayName: attr({
             compute: '_computeNameOrDisplayName',
         }),
-        partnerSeenInfos: one2many('ThreadPartnerSeenInfo', {
-            inverse: 'partner',
-            isCausal: true,
-        }),
         rtcSessions: one2many('RtcSession', {
             inverse: 'partner',
         }),
diff --git a/addons/mail/static/src/models/popover_view/popover_view.js b/addons/mail/static/src/models/popover_view/popover_view.js
index 2aba54e19ae..e7ee9028698 100644
--- a/addons/mail/static/src/models/popover_view/popover_view.js
+++ b/addons/mail/static/src/models/popover_view/popover_view.js
@@ -68,7 +68,6 @@ registerModel({
          * The record that represents the content inside the popover view.
          */
         channelInvitationForm: one2one('ChannelInvitationForm', {
-            inverse: 'popoverView',
             isCausal: true,
             readonly: true,
             required: true,
diff --git a/addons/mail/static/src/models/rtc_call_viewer/rtc_call_viewer.js b/addons/mail/static/src/models/rtc_call_viewer/rtc_call_viewer.js
index e21604dd61a..5068a35362f 100644
--- a/addons/mail/static/src/models/rtc_call_viewer/rtc_call_viewer.js
+++ b/addons/mail/static/src/models/rtc_call_viewer/rtc_call_viewer.js
@@ -366,7 +366,6 @@ registerModel({
         rtcController: one2one('RtcController', {
             default: insertAndReplace(),
             readonly: true,
-            required: true,
             inverse: 'callViewer',
             isCausal: true,
         }),
diff --git a/addons/mail/static/src/models/rtc_configuration_menu/rtc_configuration_menu.js b/addons/mail/static/src/models/rtc_configuration_menu/rtc_configuration_menu.js
index 5c54c59d82f..49003931721 100644
--- a/addons/mail/static/src/models/rtc_configuration_menu/rtc_configuration_menu.js
+++ b/addons/mail/static/src/models/rtc_configuration_menu/rtc_configuration_menu.js
@@ -87,7 +87,6 @@ registerModel({
         userSetting: one2one('UserSetting', {
             inverse: 'rtcConfigurationMenu',
             readonly: true,
-            required: true,
         }),
     },
 });
diff --git a/addons/mail/static/src/models/rtc_controller/rtc_controller.js b/addons/mail/static/src/models/rtc_controller/rtc_controller.js
index ece522ec7c8..5a9c7dbaa0e 100644
--- a/addons/mail/static/src/models/rtc_controller/rtc_controller.js
+++ b/addons/mail/static/src/models/rtc_controller/rtc_controller.js
@@ -92,7 +92,6 @@ registerModel({
             default: insertAndReplace(),
             inverse: 'rtcController',
             isCausal: true,
-            required: true,
         }),
     },
 });
diff --git a/addons/mail/static/src/models/thread_cache/thread_cache.js b/addons/mail/static/src/models/thread_cache/thread_cache.js
index 8f0b782a78e..917dc19aaca 100644
--- a/addons/mail/static/src/models/thread_cache/thread_cache.js
+++ b/addons/mail/static/src/models/thread_cache/thread_cache.js
@@ -403,6 +403,7 @@ registerModel({
         }),
         thread: one2one('Thread', {
             inverse: 'cache',
+            isCausal: true,
             readonly: true,
             required: true,
         }),
diff --git a/addons/mail/static/src/models/thread_partner_seen_info/thread_partner_seen_info.js b/addons/mail/static/src/models/thread_partner_seen_info/thread_partner_seen_info.js
index 2a17da48d0f..aa9e8161e9e 100644
--- a/addons/mail/static/src/models/thread_partner_seen_info/thread_partner_seen_info.js
+++ b/addons/mail/static/src/models/thread_partner_seen_info/thread_partner_seen_info.js
@@ -13,7 +13,6 @@ registerModel({
          * Partner that this seen info is related to.
          */
         partner: many2one('Partner', {
-            inverse: 'partnerSeenInfos',
             readonly: true,
             required: true,
         }),
diff --git a/addons/mail/static/src/models/thread_view/thread_viewer.js b/addons/mail/static/src/models/thread_view/thread_viewer.js
index 83303619b52..035a7d37537 100644
--- a/addons/mail/static/src/models/thread_view/thread_viewer.js
+++ b/addons/mail/static/src/models/thread_view/thread_viewer.js
@@ -61,10 +61,12 @@ registerModel({
     fields: {
         chatter: one2one('Chatter', {
             inverse: 'threadViewer',
+            isCausal: true,
             readonly: true,
         }),
         chatWindow: one2one('ChatWindow', {
             inverse: 'threadViewer',
+            isCausal: true,
             readonly: true,
         }),
         /**
@@ -75,6 +77,7 @@ registerModel({
         }),
         discuss: one2one('Discuss', {
             inverse: 'threadViewer',
+            isCausal: true,
             readonly: true,
         }),
         discussPublicView: one2one('DiscussPublicView', {
diff --git a/addons/mail/static/src/models/user_setting/user_setting.js b/addons/mail/static/src/models/user_setting/user_setting.js
index 02deeabf938..a857945e3aa 100644
--- a/addons/mail/static/src/models/user_setting/user_setting.js
+++ b/addons/mail/static/src/models/user_setting/user_setting.js
@@ -255,7 +255,6 @@ registerModel({
             default: insertAndReplace(),
             inverse: 'userSetting',
             isCausal: true,
-            required: true,
         }),
         /**
          * layout of the rtc session display chosen by the user
@@ -282,12 +281,5 @@ registerModel({
         voiceActiveDuration: attr({
             default: 0,
         }),
-        /**
-         * Models that represent the volume chosen by the user for each partner.
-         */
-        volumeSettings: one2many('VolumeSetting', {
-            inverse: 'userSetting',
-            isCausal: true,
-        }),
     },
 });
diff --git a/addons/mail/static/src/models/volume_setting/volume_setting.js b/addons/mail/static/src/models/volume_setting/volume_setting.js
index fef540b79b8..fd4512042cc 100644
--- a/addons/mail/static/src/models/volume_setting/volume_setting.js
+++ b/addons/mail/static/src/models/volume_setting/volume_setting.js
@@ -39,7 +39,6 @@ registerModel({
             inverse: 'volumeSetting',
         }),
         userSetting: many2one('UserSetting', {
-            inverse: 'volumeSettings',
             required: true,
         }),
         volume: attr({