Mikro ORM unique decorator is not being enforced when

Using this entity:

@Entity()
@Unique({ properties: ['case', 'csgoItem'] })
export class CaseItem {
  @PrimaryKey({ type: 'uuid', defaultRaw: 'uuid_generate_v4()' })
  id: string;

  @Property({ type: 'double precision', columnType: 'double precision' })
  probability: number;

  @ManyToOne(() => Case)
  case!: Case;

  @ManyToOne(() => CsgoItem, { cascade: [Cascade.REMOVE] })
  csgoItem!: CsgoItem;
}

When trying to insert a duplicate (based on the combination of case and csgItem, mikroORM allows me to do this. How can I make the @unique decorator work with relations?

This is a schema feature, the ORM does not enforce it anyhow, so you probably have out-of-sync schema - in other words, the unique constraint is not created.

Try running npx mikro-orm schema:update --run, or even better, adopt migrations if it's something more than just a toy project.

Few notes about the code you provided:

• probability is double, so it will be mapped to string, not number, as you can't fit a double into JS number
• no need to specify type and columnType together, not when you provide the same value in there
• the cascade: [Cascade.REMOVE] is probably not doing what you think, that is app-level cascading and means that when you delete the owning entity (CaseItem), its relations (marked with Cascade.REMOVE) will be removed too - this is the opposite of onDelete: 'cascade', which I guess you want instead (so when the target is removed - the CsgoItem entity - the owner (so Case) will be removed too