Firebase sub collections (doesn't work)

getCourses() {
    return this.afs.collection<Course>('courses').snapshotChanges()
      .map(course => {
        return course.map(a => {
          const courseData = a.payload.doc.data()
          const courseId = a.payload.doc.id;

          // get sections
          const sectionsRef = this.afs.collection(`courses/${courseId}/sections`)

          sectionsRef.snapshotChanges()
            .subscribe(section => {
              return section.map(se => {
                const sectionData = se.payload.doc.data()
                const sectionId = se.payload.doc.id;
                const theSection = { sectionId, sectionData };
                console.log('theSection', theSection)

                // get _items
                const itemsRef = this.afs.collection(`courses/${courseId}/sections/${sectionId}/items`)

                itemsRef.snapshotChanges()
                  .subscribe(item => {
                    return item.map(i => {
                      const itemData = i.payload.doc.data()
                      const itemId = i.payload.doc.id;
                      const theItem = { itemId, itemData };

                      console.log('theItem', theItem)
                      return theItem
                    });
                  })

              })

            })
        });
      })

  }