As of now, there is not out of box functionality available to create one-one relationship but there are way we can create one-one relationship in Salesforce.

1. When both object are having Lookup Relationship.

• Create a unique field on Child object.
• Write a Workflow on Child object, for any change in the lookup field, to capture the lookup field value to unique field.

Now one-one relationship is created for both.

Disadvantage of this process is –

• Creating an extra field.
• Utilizing the Unique field limit.[7 External ID fields per object]
• Using a workflow.

1. When both object are having Master Details Relationship.

• Create a roll-up summary field on Master to get the Count of Child.
• Write a validation rule on rollup summary field to check greater then 1. It will give you error if it has more than one record.

Disadvantage of this Process is –

• Feasible only if relationship is Master-Details.
• Consuming rollup summary limit.[25 Rollup Summary Fields]

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