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Room Lab 2: What does it mean when two tables are “connected”?

How Two Tables Are Connected in an Android Room App

When we build a Room database, we are basically creating a tiny version of a real SQL database inside our app.

In Lab 2, we use two tables:

Students table

Courses table

A student belongs to a course. A course can have many students.

This is called a relationship.

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⭐ 1. What does it mean when two tables are “connected”?

It does not mean the tables physically store each other.

Instead:

The Student table stores the ID of the course the student belongs to.

Example:

Table 1

Table 1

Student

GPA

courseId

Alice

3.8

Bob

2.9

Maria

There are no rows in this table

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The Course table contains:

Table 2

Table 2

courseId

courseName

Computer Science

Mathematics

There are no rows in this table

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So:

Alice → courseId = 1 → Computer Science

Maria → courseId = 2 → Mathematics

This is called a foreign key relationship.

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2. How Room “connects” the tables

Room does not magically “merge” the tables. Instead, you connect them by including the foreign key in the entity.

In our app, the connection is made by including:

val courseId: Int

inside the Student entity.

This tiny field is the entire relationship.

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3. Why this works

When you later want to find:

“All students in Course #1”

“What course does Bob belong to?”

Room can answer those questions because:

The Student row carries the courseId

The Course table has that courseId as a primary key

It’s like a matching tag or label that exists in both tables.

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4. What the app actually does with these IDs

In our Lab 2 app:

When you add a student, we hard-code courseId = 1

That means every new student belongs to Course #1

This keeps things simple for your first multi-table app.

Later in the course, you could:

Let the user choose the course with a spinner

Filter students by course

Build a Course List screen

Build a “Students in this Course” screen

But the connection always comes back to the same thing:

👉 A student carries the courseId field 👉 That value tells Room which course this student belongs to

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5. What students should understand at this level

By the end of this lab, students should know:

✔ A database can have more than one table ✔ Tables are connected by matching IDs (foreign keys) ✔ We connect tables in Room simply by adding a field in an Entity ✔ When we save a student, we also save a courseId ✔ This lets us query the database in more interesting ways later

Nothing more complicated than this is needed at this stage.

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6. The “real world” analogy

Think of:

Student = employee

Course = department

courseId = department number

Every employee record stores:

departmentId = 7

The department table says:

7 → Human Resources

That’s the entire relationship.

Room works exactly the same way.

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⭐ Final Summary

Two tables in Room are connected simply because one table stores the primary key of the other table.

Course has: courseId

Student has: courseId

Room doesn't need anything fancy. It just needs those matching numbers to relate records across tables.

This is the foundation of all relational databases — and now your students have learned it.

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