Database
[{_id:1,title:"Sub1",syllabus:{currentChapter:{heading:"H1",topics:["T1","T2"]},chapters:[{heading:"H1",topics:["T1","T2"]},{heading:"H2",topics:["T1","T2"]}]}},{_id:2,title:"Sub2",syllabus:{currentChapter:{heading:"H3",topics:[]},chapters:[{heading:"H4",topics:[]}]}}]
Query
db.collection.find({_id:1,"syllabus.chapters.heading":"H1"},{"syllabus.chapters.$":1})