The Search Engine

Querying the Database

XNAT search engine can be queried via the REST model. It can be used to retrieve a specific subset of REST resources or a table containing the relevant values. The following queries find all the subjects that are within my_project or that have an age superior to 14.

Writing the query constraints:

>>> contraints = [('xnat:subjectData/SUBJECT_ID','LIKE','%'),
...               ('xnat:subjectData/PROJECT', '=', 'my_project'),
...                'OR',
...                [('xnat:subjectData/AGE','>','14'),
...                 'AND'
...                 ]
...                ]

Combining the select statement and the where clause to retrieve the results. The result set will depend on the select which accepts paths to resources, or a specification of a table datasets with variables from the schema.

...                ['xnat:subjectData/SUBJECT_ID',
...                 'xnat:subjectData/AGE']
...         ).where(constraints)


For additional documentation on the Search Engine: - Search - SearchManager - CObject

The query syntax

Constraints for the where clause are expressed as follows:

  • A query is an unordered list that contains
    • 1 or more constraint(s)

    • 0 or more sub-queries (lists as this one)

    • 1 comparison method between the constraints

      (‘AND’ or ‘OR’)

  • A constraint is an ordered tuple that contains
    • 1 valid searchable_type/searchable_field
    • 1 operator among ‘=’, ‘<’, ‘>’, ‘<=’, ‘>=’, ‘LIKE’

Search Help

To get the searchable types and fields to put in the contraints, rows and columns parameters, use the datatypes`() method:

>>> central.inspect.datatypes(optional_filter)
[..., 'xnat:subjectData', 'xnat:projectData', 'xnat:mrSessionData',  ...]
>>> central.inspect.datatypes('xnat:subjectData', optional_filter)


How to get all the results in a query?

...                ['xnat:subjectData/SUBJECT_ID',
...                 'xnat:subjectData/AGE']).all()

How to get all the columns from a datatype?

>>> table ='xnat:subjectData').where(...)

Then to get everything:

>>> table ='xnat:subjectData').all()

Search Templates

pyxnat offers a templating feature for the search engine. The first thing is to create a template. The syntax is the same as for saving a normal search, but values in the contraints must be keys to be re-used in the future. In the following example sid is a key for the subject identifiers.

...          'template_name',
...          'xnat:subjectData',
...          ['xnat:subjectData/PROJECT', 'xnat:subjectData/SUBHECT_ID'],
...          [('xnat:subjectData/SUBJECT_ID', 'LIKE', 'sid'), 'AND'])

To use the template, the easiest way is through the search manager.

...                                    {'sid':'*5*'})
...                                    {'sid':'CENTRAL'})

It can also be used with the usual syntax with the select statement. In that case only the constraints will be used because the return data is re-defined in the select statement.

...                                              {'sid':'*5*'}))
...                ['xnat:mrSessionData/SESSION_ID']
...                ).where(template=('my_template', {'sid':'*5*'}))


This functionality hacks a bit the search saving system of XNAT. The only problem is that it will create saved searches that will be names template_something that will not work from the web interface. Do not use this feature if it is an issue for you.


For additional documentation on templates: - use_template() - saved_template() - save_template() - get_template()

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