Many-to-Many Relationship
A Many-to-Many Relationship in Salesforce links two objects where each record on one side can be associated with many records on the other side, and vice versa.
Definition
A Many-to-Many Relationship in Salesforce links two objects where each record on one side can be associated with many records on the other side, and vice versa. Salesforce does not support native many-to-many fields; the implementation uses a Junction Object: a custom object with two master-detail lookups, one to each parent. The junction object stores one row per parent-to-parent association. The classic example is Account-to-Contact via AccountContactRelation (a standard junction object), letting one Contact relate to multiple Accounts and one Account to multiple Contacts.
Many-to-Many relationships appear constantly in real-world data models. A Student takes multiple Courses, a Course enrolls multiple Students. A Contract has multiple Products, a Product appears in multiple Contracts. A Campaign reaches multiple Contacts, a Contact participates in multiple Campaigns (via CampaignMember). Building these correctly requires defining the junction object, configuring both master-detail relationships, optionally adding custom fields to the junction (Relationship Role, Start Date, Status), and configuring page layouts and related lists for both parents.
How Junction Objects implement Many-to-Many in Salesforce
The Junction Object pattern
The Junction Object is a custom object with two Master-Detail relationships, one to each parent. Each row on the junction represents one association between a specific parent-on-left and parent-on-right. The Master-Detail relationships enforce that the junction record cannot exist without both parents; deleting either parent cascades to the junction (removing the association).
Primary and secondary master-detail
When a junction object has two master-detail relationships, Salesforce designates one as the Primary Master and the other as the Secondary Master. The Primary controls record ownership, sharing, and the standard look-and-feel; the Secondary participates in the parent-child chain but does not drive ownership. Picking the right primary matters; the choice is hard to change after data exists.
Custom fields on the junction
Junction objects are full custom objects; admins can add custom fields to capture relationship attributes. Common patterns: Role (Decision Maker vs. Influencer), Date Range (when the relationship was active), Status (Primary vs. Secondary contact), Percentage (allocation share). The custom fields make the relationship semantically richer than a pure link.
Standard junction objects
Salesforce ships several standard junction objects. AccountContactRelation links Contacts to multiple Accounts. OpportunityContactRole links Contacts to Opportunities with role information. CampaignMember links Contacts and Leads to Campaigns. Pricebook Entry junctions Products to Price Books. Each standard junction has specific behavior that custom junctions cannot fully replicate.
SOQL across many-to-many
Querying across a many-to-many relationship requires traversing the junction object. Example: SELECT Name, (SELECT Contact.Name FROM AccountContactRelations) FROM Account returns each Account with its related Contacts. The subquery uses the junction''s relationship name; getting it right requires reading the junction''s API documentation.
Sharing and security
Junction objects inherit sharing from the Primary Master. A user with read access to the Primary parent record sees the junction rows; without it, they don''t. Some Industries Cloud implementations (Health Cloud Care Team Member, Financial Services Cloud Relationship) layer custom sharing on top of the junction; standard custom junctions follow standard inheritance.
Performance and limits
Junction objects are full custom objects with the same storage and query limits as any other custom object. High-volume many-to-many relationships (a Contact related to thousands of Campaigns over years) can produce millions of junction rows. Plan storage growth; consider archival processes for very large junctions.
Build a custom Many-to-Many relationship
Creating the junction object takes a few minutes; the design decisions (custom fields, primary master, related list configuration) matter more than the click path.
- Identify the two parent objects
Confirm the use case requires many-to-many. A one-to-many relationship (Account-to-Contact via Contact.AccountId) does not need a junction.
- Create the custom junction object
Setup, Object Manager, New Custom Object. Name it descriptively (Student_Course, Contact_Campaign, Account_Partner).
- Add the two master-detail relationships
On the junction, add Master-Detail to parent A, then Master-Detail to parent B. The first one is the Primary Master.
- Add custom fields if needed
Role, Date Range, Status, Percentage. Whatever attributes the relationship needs.
- Configure page layouts on the parents
Add a Related List for the junction on each parent''s page layout. Users see and create junction records from the parent record.
- Verify with sample data
Create test records on both parents and on the junction. Verify the related lists, the SOQL queries, and the sharing all work as expected.
The custom object that holds the relationships.
The Primary Master relationship.
The Secondary Master relationship.
Relationship attributes.
Required to show the related lists.
- Primary vs. Secondary Master matters. The Primary controls sharing and ownership; pick deliberately at junction creation time.
- Standard junction objects have special behavior. CampaignMember, OpportunityContactRole, AccountContactRelation cannot be replaced by custom junctions without losing platform features.
- Junction rows are full custom-object records. High-volume many-to-many relationships consume storage and query limits.
- Cascading deletes from either parent remove the junction row. Plan for this behavior; soft deletes via custom flag work better in some scenarios.
Trust & references
Cross-checked against the following references.
- Many-to-Many RelationshipsSalesforce Help
- Object RelationshipsSalesforce Help
Straight from the source - Salesforce's reference material on Many-to-Many Relationship.
- Relationships Among ObjectsSalesforce Developers
About the Author
Dipojjal Chakrabarti is a B2C Solution Architect with 29 Salesforce certifications and over 13 years in the Salesforce ecosystem. He runs salesforcedictionary.com to help admins, developers, architects, and cert/interview candidates sharpen their fundamentals. More about Dipojjal.
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