# Defining dependent table¶

In this section, we will create another manual table to store information about each experimental session. In doing so, we will see how to link two tables via dependencies, expressing how one table depends on another table.

## Defining Session table¶

Now we have defined the Mouse table to store and identify all mice in our lab, we would like to define a table called Session that will keep track of data related to all mouse experiments.

When designing a new table, it is always a good idea to start by identify by what set of attributes can be used to uniquely identify an entry in the table - thus identifying the primary key. From the descriptions given back in Building your first data pipeline, 1) an experiment session involves one mouse, 2) you can record from the same mouse on different days and 3) you could perform experiments on multiple mice per day. Putting this all together, we see that each experiment can be uniquely identified by knowing 1) on which mouse it was performed on and 2) on which day the experiment was performed on.

Given this we would want to each entry (row) of Session table to be identified by the combination of the Mouse identity and the session date. Now, notice that this means every entry of Session must have a corresponding entry in the Mouse table, reflecting the fact that you cannot perform an experiment session without a mouse! In other words, the Session table depends on the Mouse table.

DataJoint lets you formalize this relationship by expressing this dependency in the table’s definition. Let’s take a look at how Session table may be defined:

 @schema
class Session(dj.Manual):
definition = """
# experiment session
-> Mouse
session_date: date            # session date
---
experiment_setup: int         # experiment setup ID
experimenter: varchar(128)    # name of the experimenter
"""


The definition of the Session table looks very similar to that of Mouse except for one notable difference. We have this peculiar arrow notation -> Mouse in the table definition.

Note

This table uses a new data type called varchar, which stands for variable-length characters. You can use this data type to store string values up to the number specified in parenthesis. In this case, the name of the experimenter can be up to 128 characters long.

### -> for dependency specification¶

This arrow notation states that this table (Session) depends on the following table, in this case Mouse. Let’s go ahead and instantiate the table to see what the table would look like:

>>> session = Session()   # creates the table in the database
>>> session     # view the table
*mouse_id    *session_date  experiment_set experimenter
+----------+ +------------+ +------------+ +------------+

(0 tuples)


Notice how the Session table now includes the mouse_id as an attribute in its primary key despite never explicitly specifying this in the definition. By specifying that Session depends on Mouse, DataJoint automatically added the primary key attributes of the parent table (Mouse) into the dependent/child table (Session). This way, every entry in the Session table will have a mouse_id which in turn can be used to uniquely identify the mouse in the Mouse table that the experiment was performed on.

## Dependency in action¶

Now that we have the Session table defined, let’s populate this table with some data, using techniques we learned in:doc:populating-data. Remember that we can only perform experiments on a mouse that exists in the Mouse table:

>>> mouse
*mouse_id    dob            sex
+----------+ +------------+ +--------+
0            2017-03-01     M
1            2016-11-19     M
2            2016-11-20     U
5            2016-12-25     F
10           2017-01-01     F
11           2017-01-03     F
100          2017-05-12     F
(7 tuples)


Let’s insert a new session into Session table

>>> data = {
'mouse_id': 0,
'session_date': '2017-05-15',
'experiment_setup': 0,
'experimenter': 'Edgar Y. Walker'
}
>>> session.insert1(data)
>>> session
*mouse_id    *session_date  experiment_set experimenter
+----------+ +------------+ +------------+ +------------+
0            2017-05-15     0              Edgar Y. Waker
(1 tuples)


Using a valid mouse_id, we were able to successfully insert a new session. Now what would happen if try to enter a session for a mouse that does not exist? Let’s try it!

>>> data = {
'mouse_id': 9999,    # this mouse does NOT exist in Mouse table
'session_date': '2017-01-01',
'experiment_setup': 0,
'experimenter': 'Edgar Y. Walker'
}
>>> session.insert1(data)
------------------------------------------------------------------
IntegrityError                   Traceback (most recent call last)
----> 1 session.insert1(data)

...output truncated...

IntegrityError: (1452, 'Cannot add or update a child row: a foreign key constraint fails (dj_tutorial.session, CONSTRAINT session_ibfk_1 FOREIGN KEY (mouse_id) REFERENCES mouse (mouse_id) ON UPDATE CASCADE)')


Aha! We get an IntegrityError with an error message complaining about a foreign key constraint. Foreign key constraint is database lingo for the dependency that links two tables. In this case, the attribute mouse_id from dj_tutorial.session table (this is the underlying table name for the Session table object) is linked to the mouse_id attribute in dj_tutorial.mouse table (Mouse table) by a “foreign key constraint”.

The foreign key constrain ensures that a linked attributes value exists in the target table. In this case we tried to insert a row with mouse_id = 9999 which does not exist in the Mouse table and thus this violates the foreign key constraint.

## Integrity with table dependencies¶

Defining table dependencies as was done from Session to Mouse allows DataJoint to enforce data integrity by linking related table entries together. As we saw, this dependency can prevent us from entering data for an invalid target (such as trying to record a session for a mouse that doesn’t exist).

Furthermore, DataJoint uses dependencies to ensure that no dependent entires can be left “orphaned”. Let’s see what we mean by that.

### Deleting dependent entries¶

Remember the delete method back from Deleting entires? Let’s see what happens if we try to delete entries in the Mouse table that have dependent entries in Session.

Recall that Session table has an entry that points to mouse with mouse_id=0:

>>> session
*mouse_id    *session_date  experiment_set experimenter
+----------+ +------------+ +------------+ +------------+
0            2017-05-15     0              Edgar Y. Waker
(1 tuples)


Let’s first try deleting an unrelated mouse entry:

>>> (mouse & 'mouse_id = 1').delete()   # delete mouse with ID of 1
The contents of the following tables are about to be deleted:
dj_tutorial.mouse (1 tuples)
Proceed? [yes, No]:


The delete method warns you that you will be deleting one entry from Mouse, as expected. Hit Enter to cancel the deletion, and now let’s see what happens when we try to delete mouse_id=0:

>>> (mouse & 'mouse_id = 0').delete()   # delete mouse with ID of 0
The contents of the following tables are about to be deleted:
dj_tutorial.mouse (1 tuples)
dj_tutorial.session (1 tuples)
Proceed? [yes, No]:


Notice how delete method tells you that in addition to the entry in the Mouse table, an additional entry in the Session table will be deleted as well! Because there are entries in Session table that depends on the entry in Mouse we are about to delete, we have to delete both the parent and the dependent entries all together!

This cascading delete ensures that you cannot leave data entries “orphaned” - leaving behind entries that depends on non-existent parent entries. In addition to the prevention of duplication entries as we saw in Data integrity, foreign key constraint (dependency) checks and cascading deletes are a key part of DataJoint’s strength in maintaining data integrity.

## What’s next?¶

We are progressing well in our data pipeline creation adventure. We have successfully defined a new table that depends on our previous table, thereby starting to form connections or “pipes” in our data pipeline. With two linked tables, we can now perform even more exciting queries as we will cover in the next section.