Class: QgsPolymorphicRelation¶

A relation where the referenced (parent) layer is calculated based on fields from the referencing (child) layer.

A polymorphic relation consists of the same properties like a normal relation except for the referenced (parent) layer which is calculated based on one or several fields of the referencing (child) layer.

In its most simple form, the referencing layer will just insert the layer name of the referenced layer into this field. To be more precise, a polymorphic relation is a set of normal relations having the same referencing layer but having the referenced layer dynamically defined. The polymorphic setting of the layer is solved by using an expression which has to match some properties of the the referenced layer like the table name, schema, uri, layer id, …

Added in version 3.18.

Methods

`addFieldPair`	Add a field pair which is part of this relation The first element of each pair are the field names of the foreign key.
`fieldPairs`	Returns the field pairs which form this relation The first element of each pair are the field names of the foreign key.
`generateId`	Generate a (project-wide) unique id for this relation
`generateRelations`	Returns a list of generated relations, based on the currently set `referencedLayerIds()`
`hasEqualDefinition`	Compares the two `QgsRelation`, ignoring the name and the ID.
`id`	A (project-wide) unique id for this relation
`isValid`	Returns the validity of this relation.
`layerRepresentation`	Returns layer representation as evaluated string
`name`	Returns a human readable name for this relation.
`referencedFields`	Returns a list of attributes used to form the referenced fields (most likely primary key) on the referenced (parent) layer.
`referencedLayerExpression`	Returns the expression to identify the parent layer
`referencedLayerField`	Returns the field in the referencing layer where the referenced layer identifier is stored
`referencedLayerIds`	Returns a list of layer ids to be used as potential referenced layers
`referencingFields`	Returns a list of attributes used to form the referencing fields (foreign key) on the referencing (child) layer.
`referencingLayer`	Access the referencing (child) layer This is the layer which has the
`referencingLayerId`	Access the referencing (child) layer's id This is the layer which has the field(s) which point to another layer
`setId`	Set an id for this relation
`setName`	Set a name for this relation
`setReferencedLayerExpression`	Sets the expression to identify the parent layer
`setReferencedLayerField`	Sets the field in the referencing layer where the referenced layer identifier is stored
`setReferencedLayerIds`	Sets a list of layer ids to be used as potential referenced layers
`setReferencingLayer`	Set the referencing (child) layer id.
`setRelationStrength`	Sets the relation strength for all the generated normal relations
`strength`	Returns the relation strength for all the generated normal relations
`updateRelationStatus`	Updates the validity status of this relation.
`writeXml`	Writes a relation to an XML structure.

Static Methods

createFromXml

Creates a relation from an XML structure.

class qgis.core.QgsPolymorphicRelation[source]¶

Bases: object

__init__(): Default constructor. Creates an invalid relation.

__init__(context: QgsRelationContext)

Constructor with context. Creates an invalid relation.

Parameters:: context (QgsRelationContext)

__init__(other: QgsPolymorphicRelation)

Copies a relation. This makes a shallow copy, relations are implicitly shared and only duplicated when the copy is changed.

Parameters:: other (QgsPolymorphicRelation)

addFieldPair(self, referencingField: str | None, referencedField: str | None)[source]¶

Add a field pair which is part of this relation The first element of each pair are the field names of the foreign key. The second element of each pair are the field names of the matching primary key.

Parameters:

referencingField (Optional[str]) – The field name on the referencing (child) layer (FK)
referencedField (Optional[str]) – The field name on the referenced (parent) layer (PK)

static createFromXml(node: QDomNode, context: QgsReadWriteContext, relationContext: QgsRelationContext = QgsRelationContext()) → QgsPolymorphicRelation[source]¶

Creates a relation from an XML structure. Used for reading .qgs projects.

Parameters:

node (QDomNode) – The dom node containing the relation information
context (QgsReadWriteContext) – to pass project translator
relationContext (QgsRelationContext = QgsRelationContext()) – a relation context

Return type:

QgsPolymorphicRelation

Returns:

A relation

fieldPairs(self) → Dict[str, str]¶

Returns the field pairs which form this relation The first element of each pair are the field names of the foreign key. The second element of each pair are the field names of the matching primary key.

Return type:: Dict[str, str]
Returns:: The fields forming the relation

generateId(self)[source]¶: Generate a (project-wide) unique id for this relation

generateRelations(self) → List[QgsRelation]¶

Returns a list of generated relations, based on the currently set referencedLayerIds()

Return type:: List[QgsRelation]

hasEqualDefinition(self, other: QgsPolymorphicRelation) → bool[source]¶

Compares the two QgsRelation, ignoring the name and the ID.

Parameters:: other (QgsPolymorphicRelation) – The other relation
Return type:: bool
Returns:: True if they are similar

id(self) → str[source]¶

A (project-wide) unique id for this relation

Return type:: str
Returns:: The id

isValid(self) → bool[source]¶

Returns the validity of this relation. Don’t use the information if it’s not valid. A relation is considered valid if both referenced and referencig layers are valid.

Return type:: bool
Returns:: True if the relation is valid

layerRepresentation(self, layer: QgsVectorLayer | None) → str[source]¶

Returns layer representation as evaluated string

Parameters:: layer (Optional[QgsVectorLayer])
Return type:: str

name(self) → str[source]¶

Returns a human readable name for this relation. Mostly used as title for the children.