FHIRPath Model

The FHIRPathModelData struct provides runtime metadata that a FHIRPath engine needs to correctly evaluate expressions against FHIR resources. Each version package (r4, r4b, r5) exposes its own singleton instance via the FHIRPathModel() function.

The Singleton

Access the model metadata through the package-level function:

import "github.com/gofhir/models/r4/v2"

model := r4.FHIRPathModel()

The returned *FHIRPathModelData is initialized once at package load time and is safe for concurrent use. It contains all the type metadata extracted from the FHIR StructureDefinitions for that version.

Internal Data Maps

The FHIRPathModelData struct holds six internal maps, each serving a specific purpose during FHIRPath evaluation:

These maps are not exported directly. Instead, seven accessor methods provide a clean API for querying the metadata.

Accessor Methods

ChoiceTypes

Returns the permitted type codes for a polymorphic (choice) element. The path must use the base name without the type suffix.

model := r4.FHIRPathModel()

// Observation.value is a choice type (value[x])
types := model.ChoiceTypes("Observation.value")
// Returns: ["Quantity", "CodeableConcept", "string", "boolean", "integer",
//           "Range", "Ratio", "SampledData", "time", "dateTime", "Period"]

TypeOf

Returns the FHIR type code for a given element path. Primitive types use lowercase ("string", "boolean"), complex types use PascalCase ("HumanName", "CodeableConcept").

model := r4.FHIRPathModel()

t := model.TypeOf("Patient.name")
// Returns: "HumanName"

t = model.TypeOf("Patient.active")
// Returns: "boolean"

ReferenceTargets

Returns the allowed target resource types for a Reference element path.

model := r4.FHIRPathModel()

targets := model.ReferenceTargets("Observation.subject")
// Returns: ["Device", "Group", "Location", "Patient"]

ParentType

Returns the immediate parent type name in the FHIR type hierarchy.

model := r4.FHIRPathModel()

parent := model.ParentType("Patient")
// Returns: "DomainResource"

parent = model.ParentType("Age")
// Returns: "Quantity"

IsSubtype

Reports whether child is the same as or a subtype of parent by walking the type hierarchy.

model := r4.FHIRPathModel()

model.IsSubtype("Patient", "Resource")       // true
model.IsSubtype("Patient", "DomainResource") // true
model.IsSubtype("Age", "Quantity")           // true
model.IsSubtype("Quantity", "Age")           // false

ResolvePath

Resolves element paths that are defined elsewhere (e.g., backbone elements shared across types). If the path is not defined elsewhere, it returns the input unchanged.

model := r4.FHIRPathModel()

resolved := model.ResolvePath("Bundle.entry.resource")
// May return the canonical path if this element is defined on another type

IsResource

Reports whether a given type name is a FHIR resource type.

model := r4.FHIRPathModel()

model.IsResource("Patient")   // true
model.IsResource("HumanName") // false
model.IsResource("Bundle")    // true

Integration with gofhir/fhirpath

The primary use case for FHIRPathModelData is to provide type information to a FHIRPath evaluation engine. The companion library gofhir/fhirpath accepts the model via the WithModel option:

import (
    "fmt"
    "github.com/gofhir/fhirpath"
    "github.com/gofhir/models/r4/v2"
)

// Build a resource
patient := r4.NewPatientBuilder().
    SetId("example-1").
    SetBirthDate("1990-01-15").
    Build()

// Evaluate a FHIRPath expression against it
result, err := fhirpath.Evaluate(patient, "Patient.birthDate",
    fhirpath.WithModel(r4.FHIRPathModel()))
if err != nil {
    panic(err)
}
fmt.Println(result) // ["1990-01-15"]

The model enables the FHIRPath engine to:

• Resolve choice types: When an expression references Observation.value, the engine uses ChoiceTypes to know which concrete fields (valueQuantity, valueString, etc.) to check.
• Validate paths: TypeOf confirms that a path like Patient.name exists and returns HumanName.
• Navigate the type hierarchy: IsSubtype and ParentType support the FHIRPath is and as operators.
• Check reference targets: ReferenceTargets validates .resolve() calls against allowed target types.