Getting started with Newt DB

You’ll need a Postgres Database server. You can install one yourself or you can use a hosted Postgres server. You’ll need Postgres 9.5 or later.

Next, install newt.db:

pip install newt.db

You’ll eventually want to create a dedicated database and database user for Newt’s use, but if you’ve installed Postgres locally, you can just use the default database.

From Python, to get started:

>>> import newt.db
>>> connection = newt.db.connection('')

In this example, we’ve asked newt to connect to the default Postgres database. You can also supply a connection string.

The connection has a root object:

>>> connection.root
<root: >

This is the starting point for adding objects to the database.

To add data, we simply add objects to the root, directly:

>>> connection.root.first = newt.db.Object(name='My first object')

Or indirectly, as a subobject:

>>> connection.root.first.child = newt.db.Object(name='First child')

When we’re ready to save our data, we need to tell Newt to commit the changes:

>>> connection.commit()

Or, if we decide we made a mistake, we can abort any changes made since the last commit:

>>> connection.abort()

Above, we used the newt.db.Object class to create new objects. This class creates objects that behave a little bit like JavaScript objects. They’re just generic containers for properties. They’re handy for playing, and when you have a little data to store and you don’t want to bother making a custom class.

Normally, you’d create application-specific objects by subclassing Persistent [1]:

class Task(newt.db.Persistent):

   assigned = None

   def __init__(self, title, description):
       self.title = title
       self.description = description

   def assign(self, user):
       self.assigned = user

The Persistent base class helps track object changes. When we modify an object, by setting an attribute, the object is marked as changed, so that Newt will write it to the database when your application commits changes.

With a class like the one above, we can add tasks to the database:

>>> connection.root.task = Task("First task", "Explain collections")
>>> connection.commit()


Having all objects in the root doesn’t provide much organization. It’s better to create container objects. For example, we can create a task list:

class TaskList(newt.db.Persistent):

  def __init__(self):
      self.tasks = newt.db.List()

  def add(self, task):

Then when setting up our database, we’d do something like:

>>> connection.root.tasks = TaskList()
>>> connection.commit()

In the TaskList class, we using a List object. This is similar to a Python list, except that, like the Persistent base class, it tracks changes so they’re saved when your application commits changes.

Rather than supporting a single task list, we could create a list container, perhaps organized by list name:

class TaskLists(newt.db.Persistent):

    def __init__(self):
        self.lists = newt.db.BTree()

    def add(self, name, list):
        if name in self.lists:
            raise KeyError("There's already a list named", name)
        self.lists[name] = list

    def __getitem__(self, name):
        return self.lists[name]

Here, we used a BTree as the basis of our container. BTrees are mapping objects that keep data sorted on their keys.

BTrees handle very large collections well, because, when they get large, they spread their data over multiple database records, reducing the amount of data read and written and allowing collections that would be too large to keep in memory at once.

With this, building up the database could look like:

>>> connection.root.lists = TaskLists()
>>> connection.root.lists.add('docs', TaskList())
>>> connection.root.lists['docs'].add(
...     Task("First task", "Explain collections"))
>>> connection.commit()

Notice that the database is hierarchical. We access different parts of the database by traversing from object to object.


Newt leverages PostgreSQL’s powerful index and search capabilities. The simplest way to search is with a connection’s where method:

>>> tasks = connection.where("""state @> '{"title": "First task"}'""")

The search above used a Postgres JSON @> operator that tests whether its right side appears in its left side. This sort of search is indexed automatically by newt. You can also use the search method:

>>> tasks ="""
...     select * from newt where state @> '{"title": "First task"}'
...     """)

When using search, you can compose any SQL you wish, but the result must contain columns zoid and ghost_pickle. When you first use a database with Newt, it creates a number of tables, including newt:

    Table "public.newt"
    Column    |  Type  | Modifiers
 zoid         | bigint | not null
 class_name   | text   |
 ghost_pickle | bytea  |
 state        | jsonb  |
    "newt_pkey" PRIMARY KEY, btree (zoid)
    "newt_json_idx" gin (state)

The zoid column is the database primary key. Every persistent object in Newt has a unique zoid. The ghost_pickle pickle contains minimal information to, along with zoid create newt objects. The class_name column contains object’s class name, which can be useful for search. The state column contains a JSON representation of object state suitable for searching and access from other applications.

You can use PostgreSQL to define more sophisticated or application-specific indexes, as needed.

Newt has a built-in helper for defining full-text indexes on your data:

>>> connection.create_text_index('mytext', ['title', 'description', 'text'])

This creates a PL/pgSQL text-extraction function named mytext and uses it to create a text index. With the index in place, you can search it like this:

>>> tasks = connection.where("mytext(state) @@ 'explain'")

The example above finds all of the objects containing the word “explain” in their title, description, or text. We’ve assumed that these are tasks. If we wanted to make sure, we could add a “class” restriction:

>>> tasks = connection.where(
...   "mytext(state) @@ 'explain' and class_name = 'newt.demo.Task'")

Rather than creating an index directly, we can ask Newt to just return the PostgreSQL code to create them:

>>> sql = connection.create_text_index_sql(
...           'mytext', ['title', 'description', 'text'])

You can customize the returned code or just view it to see how it works.

Query errors

If you enter an invalid query and then retry, you may get an error like: “InternalError: current transaction is aborted, commands ignored until end of transaction block”. If this happens, you’ll need to abort the current transaction:

>>> connection.abort()

After that, you should be able to query again.

You can only search committed data

If you change objects, you won’t see the changes in search results until changes are committed, because data aren’t written to Postgres until the transaction is committed.

Raw queries

You can query for raw data, rather than objects using the query_data method. For example, to get a count of the various classes in your database, you could use:

>>> counts = connection.query_data("""
...     select class_name, count(*)
...     from newt
...     group by class_name
...     order by class_name
...     """)

Learning more

To learn more about Newt, see the Newt topics and the Newt topics and reference.

[1]Newt makes Persistent available as an attribute, but it’s an alias for persistent.Persistent. In fact many of the classes provided by Newt are just aliases.