generalize field handling; fields may habe multiple entries now

src/entry.ts

@@ -158,7 +158,9 @@ export class Entry {
     const editData: Record<string, FieldValueType> = {};
     for (const field of this.fields.values()) {
       if (field.edited) {
-        editData[field.element.fieldName] = field.value;
+        for (const data of field.getData()) {
+          editData[data.field] = data.value;
+        }
         field.edited = false;
       }
     }

src/fields/base.ts

@@ -24,6 +24,10 @@ export abstract class FieldBase<T> {
   public get value(): T {
     return this.entry[this.element.fieldName] as T;
   }
+
+  public getData(): Array<{ field: string; value: T | null }> {
+    return [{ field: this.element.fieldName, value: this.value }];
+  }
 }
 
 export abstract class ValueField<

src/fields/date.ts

@@ -1,3 +1,34 @@
 import { ValueField } from "./base";
 
-export default class DateField extends ValueField<string> {}
+export default class DateField extends ValueField<string> {
+  private get endDateFieldName(): string {
+    return `${this.element.uuid}_endDate`;
+  }
+
+  public get date(): string {
+    return this.value;
+  }
+
+  public get startDate(): string {
+    return this.value;
+  }
+
+  public get endDate(): string | null {
+    const val = this.entry[this.endDateFieldName];
+    if (val === null) {
+      return null;
+    }
+    return val as string;
+  }
+
+  public setEndDate(dateString: string | null): void {
+    this.entry[this.endDateFieldName] = dateString;
+    this.edited = true;
+  }
+
+  public getData(): Array<{ field: string; value: string | null }> {
+    const res = super.getData();
+    res.push({ field: this.endDateFieldName, value: this.endDate });
+    return res;
+  }
+}