A Guide to Salesforce Object Relationships

A well-structured data model is the backbone of any successful Salesforce implementation. At the heart of this model are object relationships, which create logical links between different types of data, ensuring integrity, enabling powerful features, and providing a comprehensive view of your business processes. This guide breaks down the different types of object relationships in Salesforce and explains when to use each one.

The Core Relationship Types: Lookup vs. Master-Detail

Salesforce offers two primary types of relationships to connect objects: Lookup and Master-Detail. While both link records, they have fundamental differences in behavior, data integrity, and security.

By default, an object can have a total of 40 relationship fields, a limit that can be increased to 50 upon request to Salesforce. This total is a combination of Master-Detail and Lookup relationships. An object can have a maximum of two Master-Detail relationships, so with the default limit, if you use both, you can have up to 38 lookup relationships.

Lookup Relationship: The Flexible Link

A Lookup relationship creates a simple link between two objects. Think of it as a "loose bond," where the related records are independent of each other.

Key Characteristics:

Optionality: The relationship field on a record can be left empty.
Independence: If the parent record in a lookup relationship is deleted, the child record is not deleted. There is no cascade deletion.
Flexibility: The relationship field can be edited after it has been created and can be hidden from the page layout if needed.
Limits: The number of lookup relationships you can create depends on the total relationship limit and the number of master-detail relationships on the object. By default, with a total limit of 40, you can have up to 40 lookups if you have zero master-detail relationships.

Master-Detail Relationship: The Tightly-Coupled Bond

A Master-Detail relationship creates a much stronger, parent-child dependency between objects. The child record's existence is entirely dependent on the parent (the "master") record.

Key Characteristics:

Mandatory: The relationship field on the child (detail) record cannot be left empty; it requires a parent, creating a "tight bond".
Cascade Deletion: If the parent (master) record is deleted, all of its child (detail) records are automatically deleted as well.
Inherited Security: The child record inherits all sharing and security settings from its parent record.
Immutability: The relationship field cannot be edited after the record is created, and it cannot be hidden from the page layout.
Limits: An object is limited to only two Master-Detail relationships.

Quick Comparison: Lookup vs. Master-Detail

Feature	Lookup Relationship	Master-Detail Relationship
Parent Field	Can be left empty (loose bond)	Cannot be left empty (tight bond)
Cascade Deletion	Child record is not deleted with the parent	Child record is deleted with the parent
Security & Sharing	Records have independent sharing settings	Child record inherits all settings from the parent
Field Editability	Can be edited after creation	Cannot be edited after creation
Page Layout	Can be hidden from the page layout	Cannot be hidden from the page layout
Roll-Up Summaries	Not supported (with some exceptions)	Supported
Object Limit	Up to 40 (or 50) depending on MD usage	Up to 2

Specialized Relationship Types

Beyond the two core types, Salesforce utilizes them to create more complex data structures.

Many-to-Many Relationship

When you need to link multiple records from one object to multiple records of another, you need a Many-to-Many relationship. This is achieved using a third object known as a Junction Object.

How it Works:

The junction object acts as the child object for the other two objects you want to connect.
You create a Master-Detail relationship from each of the two "parent" objects to the junction object.

Hierarchical Relationship

A Hierarchical relationship is a specific type of lookup relationship that connects records within the same object.

This is used to create structures like organizational charts or territory hierarchies.
A custom hierarchical relationship field can only be created on the User object.
The Account object has a standard hierarchical field named "Parent Account".

Features Enabled by Relationships

Roll-Up Summary Fields

This powerful feature is exclusively available for Master-Detail relationships.

A roll-up summary is a field created on the parent (master) object.
It can calculate the SUM, MIN, MAX, or COUNT of a specific field from all the related child (detail) records.

Special Standard Relationships

Some standard Salesforce objects have lookup relationships that behave like Master-Detail relationships in certain aspects.

Contact and Account: This is a lookup relationship but supports cascade deletion, meaning if you delete an Account, its related Contacts are also deleted.
Opportunity and Account: This lookup relationship supports both cascade deletion and roll-up summary fields.
Campaigns with Leads and Contacts: While your notes describe this as a lookup with master-detail features, it's more precisely facilitated through a junction object called Campaign Member. A Lead or Contact doesn't link directly to a Campaign; instead, a Campaign Member record connects the two. This structure is what allows a Campaign to support Roll-Up Summary fields to aggregate data (like counting the number of leads) from its members.

Conclusion

Choosing the correct object relationship is a critical decision in Salesforce administration and development. A Lookup relationship offers flexibility and independence, making it ideal for optional or separate connections. In contrast, a Master-Detail relationship provides a robust, dependent structure with powerful features like cascade deletion and roll-up summaries, perfect for tightly-coupled data where the child record cannot exist without the parent. By understanding these core differences and specialized use cases, you can build a scalable and efficient data model that accurately reflects your business needs.