Migrations

Neo4j does not have a set schema like relational databases, but sometimes changes to the schema and the data are required. To help with this, Neo4j.rb provides an ActiveRecord-like migration framework and a set of helper methods to manipulate both database schema and data. Just like ActiveRecord, a record of which transactions have been run will be stored in the database so that a migration is automatically only run once per environment.

Note

The migration functionality described on this page was introduced in version 8.0 of the neo4j gem.

Generators

Migrations can be created by using the built-in Rails generator:

rails generate neo4j:migration RenameUserNameToFirstName

This will generate a new file located in db/neo4j/migrate/xxxxxxxxxx_rename_user_name_to_first_name.rb

class RenameUserNameToFirstName < Neo4j::Migrations::Base
  def up
    rename_property :User, :name, :first_name
  end

  def down
    rename_property :User, :first_name, :name
  end
end

In the same way as ActiveRecord does, you should fill up the up and down methods to define the migration and (eventually) the rollback steps.

Transactions

Every migrations runs inside a transaction by default. So, if some statement fails inside a migration fails, the database rollbacks to the previous state.

However this behaviour is not always good. For instance, neo4j doesn’t allow schema and data changes in the same transaction.

To disable this, you can use the disable_transactions! helper in your migration definition:

class SomeMigration < Neo4j::Migrations::Base
  disable_transactions!

  ...
end

The schema file

When generating an empty database for your app you could run all of your migrations, but this strategy gets slower over time and can even cause issues if your older migrations become incompatible with your newer code. For this reason, whenever you run migrations a db/neo4j/schema.yml file is created which keeps track of constraints, indexes (which aren’t automatically created by constraints), and which migrations have been run. This schema file can then be loaded with the neo4j:schema:load rake task to quickly and safely setup a blank database for testing or for a new environment. While the neo4j:migrate rake task automatically creates the schema.yml file, if you ever need to generate it yourself you can use the neo4j:schema:dump rake task.

It is suggested that you check in the db/neo4j/schema.yml to your repository whenever you have new migrations.

Tasks

Neo4j.rb implements a clone of the ActiveRecord migration tasks API to migrate.

neo4j:migrate:all

Runs any pending migration.

rake neo4j:migrate:all

neo4j:migrate

An alias for rake neo4j:migrate:all.

rake neo4j:migrate:all

neo4j:migrate:up

Executes a migration given it’s version id.

rake neo4j:migrate:up VERSION=some_version

neo4j:migrate:down

Reverts a migration given it’s version id.

rake neo4j:migrate:down VERSION=some_version

neo4j:migrate:status

Prints a detailed migration state report, showing up and down migrations together with their own version id.

rake neo4j:migrate:status

neo4j:rollback

Reverts the last up migration. You can additionally pass a STEPS parameter, specifying how many migration you want to revert.

rake neo4j:rollback

neo4j:schema:dump

Reads the current database and generates a db/neo4j/schema.yml file to track constraints, indexes, and migrations which have been run (runs automatically after the neo4j:migrate task)

rake neo4j:schema:dump

neo4j:schema:load

Reads the db/neo4j/schema.yml file and loads the constraints, indexes, and migration nodes into the database. The default behavior is to only add, but an argument can be passed in to tell the task to remove any indexes / constraints that were found in the database which were not in the schema.yml file.

rake neo4j:schema:load
rake neo4j:schema:load[true] # Remove any constraints or indexes which aren't in the ``schema.yml`` file

Integrate Neo4j.rb with ActiveRecord migrations

You can setup Neo4j migration tasks to run together with standard ActiveRecord ones. Simply create a new rake task in lib/tasks/neo4j_migrations.rake:

Rake::Task['db:migrate'].enhance ['neo4j:migrate']

This will run the neo4j:migrate every time you run a rake db:migrate

Migration Helpers

#execute

Executes a pure neo4j cypher query, interpolating parameters.

execute('MATCH (n) WHERE n.name = {node_name} RETURN n', node_name: 'John')
execute('MATCH (n)-[r:`friend`]->() WHERE n.age = 7 DELETE r')

#query

An alias for Neo4j::Session.query. You can use it as root for the query builder:

query.match(:n).where(name: 'John').delete(:n).exec

#remove_property

Removes a property given a label.

remove_property(:User, :money)

#rename_property

Renames a property given a label.

rename_property(:User, :name, :first_name)

#drop_nodes

Removes all nodes with a certain label

drop_nodes(:User)

#add_label

Adds a label to nodes, given their current label

add_label(:User, :Person)

#add_labels

Adds labels to nodes, given their current label

add_label(:User, [:Person, :Boy])

#remove_label

Removes a label from nodes, given a label

remove_label(:User, :Person)

#remove_labels

Removes labels from nodes, given a label

remove_label(:User, [:Person, :Boy])

#rename_label

Renames a label

rename_label(:User, :Person)

#add_constraint

Adds a new unique constraint on a given label attribute.

Warning it would fail if you make data changes in the same migration. To fix, define disable_transactions! in your migration file.

add_constraint(:User, :name)

Use force: true as an option in the third argument to ignore errors about an already existing constraint.

#drop_constraint

Drops an unique constraint on a given label attribute.

Warning it would fail if you make data changes in the same migration. To fix, define disable_transactions! in your migration file.

drop_constraint(:User, :name)

Use force: true as an option in the third argument to ignore errors about the constraint being missing.

#add_index

Adds a new exact index on a given label attribute.

Warning it would fail if you make data changes in the same migration. To fix, define disable_transactions! in your migration file.

add_index(:User, :name)

Use force: true as an option in the third argument to ignore errors about an already existing index.

#drop_index

Drops an exact index on a given label attribute.

Warning it would fail if you make data changes in the same migration. To fix, define disable_transactions! in your migration file.

drop_index(:User, :name)

Use force: true as an option in the third argument to ignore errors about the index being missing.

#say

Writes some text while running the migration.

Ruby:
say 'Hello'
Output:
-- Hello

When passing true as second parameter, it writes it more indented.

Ruby:
say 'Hello', true
Output:
-> Hello

#say_with_time

Wraps a set of statements inside a block, printing the given and the execution time. When an Integer is returned, it assumes it’s the number of affected rows.

Ruby:
say_with_time 'Trims all names' do
  query.match(n: :User).set('n.name = TRIM(n.name)').pluck('count(*)').first
end
Output:
-- Trims all names.
   -> 0.3451s
   -> 2233 rows

#populate_id_property

Populates the uuid property (or any id_property you defined) of nodes given their model name.

populate_id_property :User

Check Adding IDs to Existing Data for more usage details.

#relabel_relation

Relabels a relationship, keeping intact any relationship attribute.

relabel_relation :old_label, :new_label

Additionally you can specify the starting and the destination node, using :from and :to.

You can specify also the :direction (one if :in, :out or :both).

Example:

relabel_relation :friends, :FRIENDS, from: :Animal, to: :Person, direction: :both

#change_relations_style

Relabels relationship nodes from one format to another.

Usage:

change_relations_style list_of_labels, old_style, new_style

For example, if you created a relationship #foo in 3.x, and you want to convert it to the 4.x+ foo syntax, you could run this.

change_relations_style [:all, :your, :labels, :here], :lower_hash, :lower

Allowed styles are:

  • :lower: lowercase string, like my_relation
  • :upper: uppercase string, like MY_RELATION
  • :lower_hash: Lowercase string starting with hash, like #my_relation