Many-to-many with references in MongoDB - Step-by-Step Execution

Concept Flow - Many-to-many with references

Create Collection A

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Create Collection B

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Insert Documents in A with refs to B

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Insert Documents in B

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Query with $lookup to join

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Get combined results

This flow shows how the students collection references the courses collection to model many-to-many relations, then how to query them together.

Execution Sample

MongoDB

db.students.insertMany([
  {_id: 1, name: "Alice", courses: [101, 102]},
  {_id: 2, name: "Bob", courses: [102, 103]}
]);
db.courses.insertMany([
  {_id: 101, title: "Math"},
  {_id: 102, title: "History"},
  {_id: 103, title: "Science"}
]);

Insert students and courses with references to model many-to-many relationships.

Execution Table

Step	Action	Collection	Document Inserted/Updated	Resulting State
1	Insert student Alice	students	{"_id":1, "name":"Alice", "courses":[101,102]}	students: [{_id:1, name:'Alice', courses:[101,102]}]
2	Insert student Bob	students	{"_id":2, "name":"Bob", "courses":[102,103]}	students: [{_id:1,...}, {_id:2, name:'Bob', courses:[102,103]}]
3	Insert course Math	courses	{"_id":101, "title":"Math"}	courses: [{_id:101, title:'Math'}]
4	Insert course History	courses	{"_id":102, "title":"History"}	courses: [{_id:101,...}, {_id:102, title:'History'}]
5	Insert course Science	courses	{"_id":103, "title":"Science"}	courses: [{_id:101,...}, {_id:102,...}, {_id:103, title:'Science'}]
6	Query students with course details using $lookup	students	Aggregation with $lookup on courses	Result: Each student document includes course info for referenced course IDs
7	Exit	-	-	All documents inserted and joined successfully

💡 All inserts done and join query executed to combine related documents.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	After Step 5	After Step 6	Final
students	[]	[{_id:1, name:'Alice', courses:[101,102]}]	[{_id:1,...}, {_id:2, name:'Bob', courses:[102,103]}]	[{_id:1,...}, {_id:2,...}]	[{_id:1,...}, {_id:2,...}]	[{_id:1,...}, {_id:2,...}]	[{_id:1,..., coursesInfo:[{_id:101,...},{_id:102,...}]}, {_id:2,..., coursesInfo:[{_id:102,...},{_id:103,...}]}]	[{_id:1,..., coursesInfo:[...]} , {_id:2,..., coursesInfo:[...]}]
courses	[]	[]	[]	[{_id:101, title:'Math'}]	[{_id:101,...}, {_id:102, title:'History'}]	[{_id:101,...}, {_id:102,...}, {_id:103, title:'Science'}]	[{_id:101,...}, {_id:102,...}, {_id:103,...}]	[{_id:101,...}, {_id:102,...}, {_id:103,...}]

Key Moments - 3 Insights

Why do we store arrays of course IDs in student documents instead of embedding full documents?

Storing only IDs keeps documents small and avoids duplication. The execution_table step 6 shows how $lookup joins data at query time.

How does $lookup know which documents to join?

$lookup matches the array of IDs in students.courses with courses._id as shown in execution_table step 6, combining related documents.

What happens if a referenced course ID does not exist?

The $lookup will return an empty array for that course reference, so the student document will have fewer joined course details.

Visual Quiz - 3 Questions

Look at the execution_table at step 2. What is the courses array for student Bob?

A [101, 102]

B [102, 103]

C [101, 103]

D []

💡 Hint

Check the 'Document Inserted/Updated' column at step 2 in execution_table.

At which step do we insert the course titled 'History'?

A Step 4

B Step 3

C Step 5

D Step 6

💡 Hint

Look at the 'Action' and 'Document Inserted/Updated' columns in execution_table.

If we remove course ID 102 from Alice's courses array, what changes in variable_tracker after step 6?

A Alice's coursesInfo will be empty

B Alice's coursesInfo will include courses 101 and 102 details

C Alice's coursesInfo will include only course 101 details

D No change in coursesInfo

💡 Hint

Refer to variable_tracker row for 'students' after step 6 showing coursesInfo arrays.

Concept Snapshot

Many-to-many with references in MongoDB:
- Store arrays of course IDs in student documents.
- Use $lookup aggregation to join related documents.
- Keeps data normalized and queries flexible.
- Example: students have courses array of course IDs.
- Query joins students with course details by matching IDs.

Full Transcript

This visual execution shows how to model many-to-many relationships in MongoDB using references. We create two collections, students and courses. Each student document stores an array of course IDs they attend. Each course document stores its details. We insert sample students and courses. Then we use the $lookup aggregation stage to join students with their course documents by matching course IDs. The execution table traces each insert and the join query step-by-step. The variable tracker shows how the students and courses collections grow and how the joined results include course details. Key moments clarify why we store IDs instead of full documents and how $lookup works. The quiz tests understanding of the data at each step. This method keeps data normalized and queries efficient for many-to-many relations.