Tables in the initial portions of the pipeline are populated from outside the pipeline. In subsequent steps, computations are performed automatically by the DataJoint pipeline.

Computed tables belong to one of the two auto-populated Data Tiers: dj.Imported and dj.Computed. DataJoint does not enforce the distinction between imported and compputed tables: the difference is purely semantic, a convention for developers to follow. If populating a table requires access to external files such as raw storage that is not part of the database, the table is designated as imported. Otherwise, it is computed.


Auto-populated tables are defined and queried exactly as other tables such as Manual Tables, for example. Their data definition follows the same Definition syntax.

For auto-populated tables, data should never be entered using Insert directly. Instead, these tables must define the callback method makeTuples(self, key) in MATLAB _make_tuples(self, key). The insert method then can only be called on self inside this callback method.

Consider the following example:

Imagine that there is a table test.Image that contains 2D grayscale images in its image attribute. Let us define the computed table, test.FilteredImage that filters the image in some way and saves the result in its filtered_image attribute.

The class will be defined as follows.

matlab MATLAB

# Filtered image
-> test.Image
filtered_image : longblob

classdef FilteredImage < dj.Computed
        function makeTuples(self, key)
            img = fetch1(test.Image & key, 'image');
            key.filtered_image = myfilter(img);

python Python

class FilteredImage(dj.Computed):
    definition = """
    # Filtered image
    -> Image
    filtered_image : longblob

    def _make_tuples(self, key):
        img = (test.Image() & key).fetch1['image']
        key['filtered_image'] = myfilter(img)

The make_tuples method received one argument: the key of type struct in MATLAB and dict in Python. The key represents the partially filled tuple, usually already containing Primary Key attributes.

Inside the callback, three things always happen:

  1. Fetch data from tables upstream in the pipeline using the key for Restriction.
  2. The missing attributes are computed and added to the fields allredy in key.
  3. The entire tuple is inserted into self.

make_tuples may populate multiple tuples in one call when key does not specify the entire primary key of the populated table.


The inherited populate method of dj.Imported and dj.Computed automatically calls make_tuples for every key for which the auto-populated table is missing data.

The FilteredImage table can be populated as

python Python


matlab MATLAB


Note that it is not necessary which data needs to be computed. DataJoint will call make_tuples, one-by-one, for every key in Image for which FilteredImage has not yet been computed.

Chains of auto-populated tables form computational pipelines in DataJoint.