title: combine states of 2 redux stores
body: <p>I have 2 stores, one about properties and one about owners.
  Each property is owned by one or more owners, and I have to group the properties by owner. So like a map structure Map&lt;Owner, Property[]&gt;.</p>
  <p>How to I retrieve both objects you ask? Redux selectors!</p>
  <p>For properties I have a facade which has a selector</p>
  <pre><code>properties$ = this.store$.select(PropertySummarySelectors.selectProperties);
  </code></pre>
  <p>This is what is used now, to show a list of all properties on a page, now we have to add a tab, which will group the properties by owner. Why doesn't property have data about the owner, the owner is human and we can't store personal information on our side, privacy stuff.</p>
  <p>In another store for owners, we also have a facade, which has a selector that I need</p>
  <pre><code> ownersOfProperty$ = (propertyId) =&gt; this.store$.select(OwnerSelectors.selectOwnerForProperty, propertyId);
  </code></pre>
  <p>This will return a list of owners for the given property using the following selector:</p>
  <pre><code>  export const selectOwnerForProperty = createSelector(
      selectOwnerForProperty,
      (ownersPerProperty, propertyId) =&gt; ownersPerProperty[propertyId] || []
    );
  </code></pre>
  <p>So when calling the OwnerFacade, I get an Observable.</p>
  <p>My idea was, when calling the property$ selector, for every object in that list, call the other facade to get the owners.</p>
  <pre><code>this.property$ = this.propertySummaryFacade.properties$.pipe(
          map(properties =&gt; properties.map(
              async property =&gt; {
                property.owners = await this.ownerFacade.ownerOfProperty$(property.id);
                return property;
              })
          )
      );
  </code></pre>
  <p>It doesn't resolve it, it looks ugly. How to I combine these 2 states for 2 stores correctly, without extending my Property object on the backend side, which is again not easy due to privacy regulation.</p>