Don't allow singletons to be selected in m2a context

Closes #4406

app/src/components/v-select/v-select.vue

@@ -62,6 +62,7 @@
 							:inputValue="value || []"
 							:label="item.text"
 							:value="item.value"
+							:disabled="item.disabled"
 							@change="$emit('input', $event.length > 0 ? $event : null)"
 						/>
 					</v-list-item-content>
@@ -121,6 +122,7 @@
 import { defineComponent, PropType, computed, toRefs, Ref } from '@vue/composition-api';
 import i18n from '@/lang';
 import { useCustomSelection, useCustomSelectionMultiple } from '@/composables/use-custom-selection';
+import { get } from 'lodash';
 
 type ItemsRaw = (string | any)[];
 type InputValue = string[] | string;
@@ -143,6 +145,10 @@ export default defineComponent({
 			type: String,
 			default: null,
 		},
+		itemDisabled: {
+			type: String,
+			default: 'disabled',
+		},
 		value: {
 			type: [Array, String, Number, Boolean] as PropType<InputValue>,
 			default: null,
@@ -218,10 +224,10 @@ export default defineComponent({
 					if (item.divider === true) return { divider: true };
 
 					return {
-						text: item[props.itemText],
-						value: item[props.itemValue],
-						icon: item[props.itemIcon],
-						disabled: item.disabled,
+						text: get(item, props.itemText),
+						value: get(item, props.itemValue),
+						icon: get(item, props.itemIcon),
+						disabled: get(item, props.itemDisabled),
 					};
 				});
 

app/src/modules/settings/routes/data-model/field-detail/components/relationship-m2a.vue

@@ -66,6 +66,7 @@
 					:items="availableCollections"
 					item-value="collection"
 					item-text="name"
+					item-disabled="meta.singleton"
 					multiple
 					v-model="relations[1].one_allowed_collections"
 					:multiple-preview-threshold="0"