The validation context

The validation context#

The core structure of the validator is the context, a namespace that aggregates properties of the dataset (the dataset variable, above) and the current file being validated.

Its type can be described as follows:

Context: {
  // Dataset properties
  dataset: {
    dataset_description: object
    datatypes: string[]
    modalities: string[]
    // Lists of subjects as discovered in different locations
    subjects: {
        sub_dirs: string[]
        participant_id: string[]
        phenotype: string[]
    }
  }

  // Properties of the current subject
  subject: {
      // Lists of sessions as discovered in different locations
      sessions: {
        ses_dirs: string[]
        session_id: string[]
        phenotype: string[]
      }
  }

  // Path properties
  path: string
  entities: object
  datatype: string
  suffix: string
  extension: string
  // Inferred property
  modality: string

  // Inheritance principle constructions
  sidecar: object
  associations: {
    // Paths and properties of files associated with the current file
    aslcontext: { path: string, n_rows: integer, volume_type: string[] }
    ...
  }

  // Content properties
  size: integer

  // File type-specific content properties
  columns: object
  gzip: object
  json: object
  nifti_header: object
  ome: object
  tiff: object
}

To take an example, in a minimal dataset containing only a single subject’s T1-weighted image, the context for that image might be:

dataset:
  dataset_description:
    Name: "Example dataset"
    BIDSVersion: "1.10.0"
    DatasetType: "raw"
  datatypes: ["anat"]
  modalities: ["mri"]
  subjects:
    sub_dirs: ["sub-01"]
    participant_id: null
    phenotype: null

subject:
  sessions: { ses_dirs: null, session_id: null, phenotype: null }

path: "/sub-01/anat/sub-01_T1w.nii.gz"
entities:
  subject: "01"
datatype: "anat"
suffix: "T1w"
extension: ".nii.gz"
modality: "mri"

sidecar:
  MagneticFieldStrength: 3
  ...
associations: {}

size: 22017017
nifti_header:
  dim: 3
  voxel_sizes: [1, 1, 1]
  ...

Fields from this context can be queried using object dot notation. For example, sidecar.MagneticFieldStrengh has the integer value 3, and entities.subject has the string value "01". This permits the use of boolean expressions, such as sidecar.RepetitionTime == nifti_header.pixdim[4].

As the validator validates each file in turn, it constructs a new context. The dataset property remains constant, while a new subject property is constructed when inspecting a new subject directory, and the remaining properties are constructed for each file, individually.

Context definition#

The validation context is largely dictated by the schema, and the full type generated from the schema definition can be found in jsr:@bids/schema/context.

Context construction#

The construction of a validation context is where BIDS concepts are implemented. Again, this is easiest to explain with pseudocode:

def buildFileContext(dataset, file):
    context = namespace()
    context.dataset = dataset
    context.path = file.path
    context.size = file.size

    fileParts = parsePath(file.path)
    context.entities = fileParts.entities
    context.datatype = fileParts.datatype
    context.suffix = fileParts.suffix
    context.extension = fileParts.extension

    context.subject = buildSubjectContext(dataset, context.entities.subject)

    context.sidecar = loadSidecar(file)
    context.associations = namespace({
        association: loadAssociation(file, association)
        for association in associationTypes(file)
    })

    if isTSV(file):
        context.columns = loadColumns(file)
    if isNIfTI(file):
        context.nifti_header = loadNiftiHeader(file)
    ...  # And so on

    return context

The heavy lifting is done in parsePath, loadSidecar and loadAssociation. parsePath is relatively simple, but loadSidecar and loadAssociation implement the BIDS Inheritance Principle.

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