greenrobot/greenDAO · error · RuntimeException

Currently only single FK columns are supported:

Error message

Currently only single FK columns are supported: 

What it means

ToOne.init3ndPass() builds the foreign-key column list for a to-one relation. greenDAO's generator only supports a single FK column: if fkProperties has more than one entry, or the target entity has no PK property at all, it throws this RuntimeException naming the relation.

Solutions

1. Use a single-column primary key on the target entity and a single FK property on the source
2. Remove extra fkProperties so only one remains, or use entity.addToOne(targetEntity, singleFkProperty)
3. If you need composite references, use a ToMany or model the join explicitly with an intermediate entity
4. Ensure the target entity has a PK (addIdProperty().primaryKey()) before the relation's 3rd pass

Example fix

// before
ToOne toOne = order.addToOne(customer, null);
toOne.setFkProperties(custRegionProp, custCodeProp); // 2 FKs
// after
ToOne toOne = order.addToOne(customer, order.getCustomerIdProperty()); // single FK

if (targetEntity.getPkProperty() == null || fkProps.size() > 1) throw new IllegalStateException("ToOne requires single FK and single-PK target");

function singleFkValid(fkProps, target) { return Array.isArray(fkProps) && fkProps.length === 1 && target.getPkProperty() != null; }

try { schema.generate(); } catch (RuntimeException e) { if (e.getMessage().startsWith("Currently only single FK columns")) { simplifyRelation(e.getMessage()); } else { throw e; } }

When it happens

Trigger: Calling setFkProperties with more than one property on a ToOne; or creating a ToOne to a target entity that has no primary key (getPkProperty() returns null); a ToOne defined against a composite-key target.

Common situations: Modeling a to-one relation to an entity with a composite primary key; copying ToMany composite-key patterns onto a ToOne; target entity defined without addIdProperty().primaryKey().

Understand the failure class

Background: UnsupportedOperationException and "is not supported" errors: when a library deliberately refuses a call — this error's family across 30 libraries.

Related errors

• Property does not exist in
• Type is already non-primitive
• No mapping for
• Source entity has no primary key, but we need it for
• Source properties do not match target properties:

AI-assisted analysis of greenrobot/greenDAO@0bbb338e17 (2026-09-08). Data as JSON: /api/errors/832a685944c3f3d2. Report an issue: GitHub.

Appendix: source

Thrown at DaoGenerator/src/org/greenrobot/greendao/generator/ToOne.java:93

    public boolean isUseFkProperty() {
        return useFkProperty;
    }

    void init2ndPass() {
        if (name == null) {
            char[] nameCharArray = targetEntity.getClassName().toCharArray();
            nameCharArray[0] = Character.toLowerCase(nameCharArray[0]);
            name = new String(nameCharArray);
        }

    }

    /** Constructs fkColumns. Depends on 2nd pass of target key properties. */
    void init3ndPass() {

        Property targetPkProperty = targetEntity.getPkProperty();
        if (fkProperties.length != 1 || targetPkProperty == null) {
            throw new RuntimeException("Currently only single FK columns are supported: " + this);
        }

        Property property = fkProperties[0];
        PropertyType propertyType = property.getPropertyType();
        if (propertyType == null) {
            propertyType = targetPkProperty.getPropertyType();
            property.setPropertyType(propertyType);
            // Property is not a regular property with primitive getters/setters, so let it catch up
            property.init2ndPass();
            property.init3ndPass();
        } else if (propertyType != targetPkProperty.getPropertyType()) {
            System.err.println("Warning to-one property type does not match target key type: " + this);
        }
        resolvedKeyJavaType[0] = schema.mapToJavaTypeNullable(propertyType);
        resolvedKeyUseEquals[0] = checkUseEquals(propertyType);
    }

    protected boolean checkUseEquals(PropertyType propertyType) {

View on GitHub (pinned to 0bbb338e17)