Design the Data Model
By the end of this lesson, you'll be able to:
- Design custom objects and fields for a library system
- Decide which parts of the system belong in the data model vs in Apex service classes
- Apply the "find the nouns" technique from Module 12 to Salesforce object design
Prerequisites: Module 21: "SOSL"
What we're building
Over this module you'll build a small library system, working against real Salesforce custom objects, that:
- Tracks books in a catalog.
- Records checkouts and returns.
- Lets a librarian query and search the catalog.
- Handles real-world edge cases — a book that's already checked out, a member trying to return a book they didn't borrow.
This is the course's first project working against actual sObjects rather than pure in-memory classes — every SOQL, DML, and SOSL lesson from Modules 18-21 gets put to real use here.
Designing the data model
Applying Module 12's "find the nouns" technique to "a library tracks books, and members check books out and return them":
Book__c
- Title__c (Text)
- Author__c (Text)
- ISBN__c (Text, External ID)
- Is_Checked_Out__c (Checkbox)
Checkout__c
- Book__c (Lookup to Book__c)
- Member_Name__c (Text)
- Checkout_Date__c (Date)
- Return_Date__c (Date, blank until returned)This is Module 18's custom object and custom field patterns directly applied — every field name ending in __c, exactly as covered there.
What belongs in the data model vs in Apex
The data model above only stores facts — it doesn't decide "is this book available to check out?" That decision belongs in Apex code (the next lesson builds a LibraryService class for exactly this), which reads the stored facts and applies business rules to them. Keeping the data model itself simple and the logic in Apex is a deliberate separation, one this course revisits more formally in a much later Enterprise Architecture module.
Exercise
As a comment, decide whether "the number of days a book was checked out" belongs as a stored field or as something calculated in Apex from Checkout_Date__c and Return_Date__c.
Show hint
Think about whether storing it risks it going stale.
Design the Data Model Quiz
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Summary
Before writing any code, this lesson designs the two custom objects a library system needs — Book__c and Checkout__c — and decides which responsibilities belong in the data model versus in Apex.