How to narrow a record field by another field#

This guide makes a sample field follow a dataset field on the same form. You would do this when a person should pick a sample from the dataset they have just chosen, or see that dataset’s samples first.

Steps#

  1. Declare the field the other one follows, with the scope the form needs.

    from django import forms

from fairdm.contrib.autocomplete.fields import DatasetChoiceField, SampleChoiceField


class PlaceSampleForm(forms.Form):
    dataset = DatasetChoiceField(scope="edit")

  2. Add the sample field to the form, with the name of the first field as depends_on.

    class PlaceSampleForm(forms.Form):
    dataset = DatasetChoiceField(scope="edit")
    sample = SampleChoiceField(depends_on="dataset")

    The field then offers only the samples of the chosen dataset.

  3. To list those samples first and still offer the rest, pass the name as prefer instead.

    class PlaceSampleForm(forms.Form):
    dataset = DatasetChoiceField(scope="edit")
    sample = SampleChoiceField(prefer="dataset")

    The samples of the chosen dataset come first, under a heading of their own. The field still accepts every sample the person may view. Use this when a record may refer to a sample in another dataset, as a measurement may.

  4. Build the form inside a request.

    form = PlaceSampleForm(request.POST or None)

The result#

With depends_on, the browser searches only the samples of the chosen dataset and clears the sample choice when the dataset changes. On submit, the field reads the dataset from the form’s own data and refuses a sample of any other dataset with invalid_choice. When no dataset is chosen, the field is not narrowed.

depends_on and prefer accept "dataset" or "polymorphic_ctype", for samples and measurements. Any other name raises ValueError when the field is created. See Record fields.

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