Many-Many relationship in MongoDB

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A) (Many-to-Many) Many-to-Many Relationships with Document Reference
This is the most normalized and best form to represent many to many relationship data model.
Create student collection and COURSE collection.
Then create STUDENT_COURSE collection which contains reference to student and COURSE collection.
1) Create collections in MongoDB >
STEP 1.1) create and insert in STUDENT collection >
db.STUDENT.insert({
  "_id": 1,
  "FIRST_NAME": "Ankit"
})
db.STUDENT.insert({
  "_id": 2,
  "FIRST_NAME": "Sam"
})
STEP 1.2) create and insert in COURSE collection >
db.COURSE.insert({
  "_id": 11,
  "COURSE_NAME": "Hindi"
})
db.COURSE.insert({
  "_id": 12,
  "COURSE_NAME": "English"
})
STEP 1.3) create and insert in STUDENT_COURSE collection >
db.STUDENT_COURSE.insert({
  "_id": 21,
  "STUDENT_ID": 1 ,
  "COURSE_ID": 11
})
db.STUDENT_COURSE.insert({
  "_id": 22,
  "STUDENT_ID": 1 ,
  "COURSE_ID": 12
})
db.STUDENT_COURSE.insert({
  "_id": 23,
  "STUDENT_ID": 2 ,
  "COURSE_ID": 11
})
db.STUDENT_COURSE.insert({
  "_id": 24,
  "STUDENT_ID": 2 ,
  "COURSE_ID": 12,
})


2) Now, let’s see above many-many relationship of mongoDB collections in RDBMS (relational database) >
2.1) Many-Many Relationship - Table structure in RDBMS >
2.2) Sql script to create above tables in RDBMS (in oracle) >
create table STUDENT (ID number PRIMARY KEY,
                   FIRST_NAME varchar2(22));

create table COURSE (ID number PRIMARY KEY,
                   COURSE_NAME varchar2(22) );

create table STUDENT_COURSE (ID number PRIMARY KEY,
                   STUDENT_ID number,
                   COURSE_ID number,
                   FOREIGN KEY (STUDENT_ID) REFERENCES STUDENT (ID),
                   FOREIGN KEY (COURSE_ID) REFERENCES COURSE (ID) );
2.2.3) Let’s see tables after inserting data in RDBMS >
Here, Many students study many courses.
3 > Now, let’s read/query/find in above (2.1) MongoDB collections >
Query 3.1 > Query to find and show all students
db.STUDENT.find().pretty()
Output>
{ "_id" : 1, "FIRST_NAME" : "Ankit" }
{ "_id" : 2, "FIRST_NAME" : "Sam" }
Query 3.2 > Query to find and show all students
db.COURSE.find().pretty()
Output>
{ "_id" : 11, "COURSE_NAME" : "Hindi" }
{ "_id" : 12, "COURSE_NAME" : "English" }
Query 3.3 > Query to find and show all student_courses
db.STUDENT_COURSE.find().pretty()
Output>
{ "_id" : 21, "STUDENT_ID" : 1, "COURSE_ID" : 11 }
{ "_id" : 22, "STUDENT_ID" : 1, "COURSE_ID" : 12 }
{ "_id" : 23, "STUDENT_ID" : 2, "COURSE_ID" : 11 }
{ "_id" : 24, "STUDENT_ID" : 2, "COURSE_ID" : 12 }
B) Now let’s cover above point - (Inserting related documents in same collection) - Many-to-Many Relationships with Document Reference >
create and insert in STUDENT collection (Also insert related data i.e. data (documents) of COURSE and STUDENT_COURSE in STUDENT collection) >
db.STUDENT.insert({
  "_id": 1,
  "FIRST_NAME": "Ankit"
})
db.STUDENT.insert({
  "_id": 2,
  "FIRST_NAME": "Sam"
})


db.STUDENT.insert({
  "_id": 11,
  "COURSE_NAME": "Hindi"
})
db.STUDENT.insert({
  "_id": 12,
  "COURSE_NAME": "English"
})


db.STUDENT.insert({
  "_id": 21,
  "STUDENT_ID": 1 ,
  "COURSE_ID": 11
})
db.STUDENT.insert({
  "_id": 22,
  "STUDENT_ID": 1 ,
  "COURSE_ID": 12
})
db.STUDENT.insert({
  "_id": 23,
  "STUDENT_ID": 2 ,
  "COURSE_ID": 11
})
db.STUDENT.insert({
  "_id": 24,
  "STUDENT_ID": 2 ,
  "COURSE_ID": 12,
})



C) Summary -
So in this mongoDB tutorial we learned how to create and manage Many-Many relationship in MongoDB.
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