wagtail/docs/advanced_topics/streamfield_migrations.md

(streamfield_migrations)=

StreamField migrations

(streamfield_migrating_richtext)=

Migrating RichTextFields to StreamField

If you change an existing RichTextField to a StreamField, the database migration will complete with no errors, since both fields use a text column within the database. However, StreamField uses a JSON representation for its data, so the existing text requires an extra conversion step to become accessible again. For this to work, the StreamField needs to include a RichTextBlock as one of the available block types. Create the migration as normal using ./manage.py makemigrations, then edit it as follows (in this example, the 'body' field of the demo.BlogPage model is being converted to a StreamField with a RichTextBlock named rich_text):

import json

from django.core.serializers.json import DjangoJSONEncoder
from django.db import migrations

import wagtail.blocks
import wagtail.fields


def convert_to_streamfield(apps, schema_editor):
    BlogPage = apps.get_model("demo", "BlogPage")
    for page in BlogPage.objects.all():
        page.body = json.dumps(
            [{"type": "rich_text", "value": page.body}],
            cls=DjangoJSONEncoder
        )
        page.save()


def convert_to_richtext(apps, schema_editor):
    BlogPage = apps.get_model("demo", "BlogPage")
    for page in BlogPage.objects.all():
        if page.body:
            stream = json.loads(page.body)
            page.body = "".join([
                child["value"] for child in stream
                if child["type"] == "rich_text"
            ])
            page.save()


class Migration(migrations.Migration):

    dependencies = [
        # leave the dependency line from the generated migration intact!
        ("demo", "0001_initial"),
    ]

    operations = [
        migrations.RunPython(
            convert_to_streamfield,
            convert_to_richtext,
        ),

        # leave the generated AlterField intact!
        migrations.AlterField(
            model_name="BlogPage",
            name="body",
            field=wagtail.fields.StreamField(
                [("rich_text", wagtail.blocks.RichTextBlock())],
            ),
        ),
    ]

Note that the above migration will work on published Page objects only. If you also need to migrate draft pages and page revisions, then edit the migration as in the following example instead:

import json

from django.contrib.contenttypes.models import ContentType
from django.core.serializers.json import DjangoJSONEncoder
from django.db import migrations

import wagtail.blocks
import wagtail.fields


def page_to_streamfield(page):
    changed = False
    try:
        json.loads(page.body)
    except ValueError:
        page.body = json.dumps(
            [{"type": "rich_text", "value": page.body}],
        )
        changed = True
    else:
        # It's already valid JSON. Leave it.
        pass

    return page, changed


def pagerevision_to_streamfield(revision_data):
    changed = False
    body = revision_data.get("body")
    if body:
        try:
            json.loads(body)
        except ValueError:
            revision_data["body"] = json.dumps(
                [{
                    "value": body,
                    "type": "rich_text"
                }],
                cls=DjangoJSONEncoder)
            changed = True
        else:
            # It's already valid JSON. Leave it.
            pass
    return revision_data, changed


def page_to_richtext(page):
    changed = False
    if page.body:
        try:
            body_data = json.loads(page.body)
        except ValueError:
            # It's not apparently a StreamField. Leave it.
            pass
        else:
            page.body = "".join([
                child["value"] for child in body_data
                if child["type"] == "rich_text"
            ])
            changed = True

    return page, changed


def pagerevision_to_richtext(revision_data):
    changed = False
    body = revision_data.get("body", "definitely non-JSON string")
    if body:
        try:
            body_data = json.loads(body)
        except ValueError:
            # It's not apparently a StreamField. Leave it.
            pass
        else:
            raw_text = "".join([
                child["value"] for child in body_data
                if child["type"] == "rich_text"
            ])
            revision_data["body"] = raw_text
            changed = True
    return revision_data, changed


def convert(apps, schema_editor, page_converter, pagerevision_converter):
    BlogPage = apps.get_model("demo", "BlogPage")
    content_type = ContentType.objects.get_for_model(BlogPage)
    Revision = apps.get_model("wagtailcore", "Revision")

    for page in BlogPage.objects.all():

        page, changed = page_converter(page)
        if changed:
            page.save()

        for revision in Revision.objects.filter(
            content_type_id=content_type.pk, object_id=page.pk
        ):
            revision_data = revision.content
            revision_data, changed = pagerevision_converter(revision_data)
            if changed:
                revision.content = revision_data
                revision.save()


def convert_to_streamfield(apps, schema_editor):
    return convert(apps, schema_editor, page_to_streamfield, pagerevision_to_streamfield)


def convert_to_richtext(apps, schema_editor):
    return convert(apps, schema_editor, page_to_richtext, pagerevision_to_richtext)


class Migration(migrations.Migration):

    dependencies = [
        # leave the dependency line from the generated migration intact!
        ("demo", "0001_initial"),
        ("wagtailcore", "0076_modellogentry_revision"),
    ]

    operations = [
        migrations.RunPython(
            convert_to_streamfield,
            convert_to_richtext,
        ),

        # leave the generated AlterField intact!
        migrations.AlterField(
            model_name="BlogPage",
            name="body",
            field=wagtail.fields.StreamField(
                [("rich_text", wagtail.blocks.RichTextBlock())],
            ),
        ),
    ]

(streamfield_data_migrations)=

StreamField data migrations

Wagtail provides a set of utilities for creating data migrations on StreamField data. These are exposed through the modules:

• wagtail.blocks.migrations.migrate_operation
• wagtail.blocks.migrations.operations

• wagtail.blocks.migrations.utils

  An add-on package [wagtail-streamfield-migration-toolkit](https://github.com/wagtail/wagtail-streamfield-migration-toolkit) is available, additionally providing limited support for auto-generating migrations.
  

Why are data migrations necessary?

If you change the block definition of a StreamField on a model that has existing data, you may have to manually alter that data to match the new format.

A StreamField is stored as a single column of JSON data in the database. Blocks are stored as structures within the JSON, and can be nested. However, as far as Django is concerned when generating schema migrations, everything inside this column is just a string of JSON data. The database schema doesn’t change - regardless of the content/structure of the StreamField - since it is the same field type before and after any change to the StreamField's blocks. Therefore whenever changes are made to StreamFields, any existing data must be changed into the new required structure, typically by defining a data migration. If the data is not migrated, even a simple change like renaming a block can result in old data being lost.

Generally, data migrations are performed manually by making an empty migration file and writing the forward and backward functions for a RunPython command. These functions handle the logic for taking the previously saved JSON representation and converting it into the new JSON representation needed. While this is fairly straightforward for simple changes (such as renaming a block), this can easily get very complicated when nested blocks, multiple fields, and revisions are involved.

To reduce boilerplate, and the potential for errors, wagtail.blocks.migrations provides the following:

• utilities to recurse through stream data structures and apply changes; and
• operations for common use cases like renaming, removing and altering values of blocks.

(streamfield_migration_basic_usage)=

Basic usage

Suppose we have a BlogPage model in an app named blog, defined as follows:

class BlogPage(Page):
    content = StreamField([
        ("stream1", blocks.StreamBlock([
            ("field1", blocks.CharBlock())
        ])),
    ])

After running the initial migrations and populating the database with some records, we decide to rename field1 to block1.

class BlogPage(Page):
    content = StreamField([
        ("stream1", blocks.StreamBlock([
            ("block1", blocks.CharBlock())
        ])),
    ])

Even though we changed the name to block1 in our StreamField definition, the actual data in the database will not reflect this. To update existing data, we need to create a data migration.

First we create an empty migration file within the app. We can use Django's makemigrations command for this:

python manage.py makemigrations --empty blog

which will generate an empty migration file which looks like this:

# Generated by Django 4.0.3 on 2022-09-09 21:33

from django.db import migrations


class Migration(migrations.Migration):

    dependencies = [...]

    operations = [
    ]

We need to make sure that either this migration or one of the migrations it depends on has the Wagtail core migrations as a dependency, since the utilities need the migrations for the Revision models to be able to run.

    dependencies = [
        ('wagtailcore', '0069_log_entry_jsonfield'),
        ...
    ]

(if the project started off with Wagtail 4, '0076_modellogentry_revision' would also be fine)

Next we need a migration operation which Django will run to make our changes. If we weren't using the provided utilities, we would use a migrations.RunPython operation, and we would define what data (model, field etc.) we want and how we want to change that data in its forward (function) argument.

Instead, we have a migrate_operation.MigrateStreamData operation which will handle accessing the relevant data for us. We need to specify the app name, model name and field name for the relevant StreamField as shown below.

from django.db import migrations

from wagtail.blocks.migrations.migrate_operation import MigrateStreamData

class Migration(migrations.Migration):

    dependencies = [...]

    operations = [
        MigrateStreamData(
            app_name="blog",
            model_name="BlogPage",
            field_name="content",
            operations_and_block_paths=[...]
        ),
    ]

In a StreamField, accessing just the field is not enough, since we will typically need to operate on specific block types. For this, we define a block path which points to that specific block path within the StreamField definition to obtain the specific data we need. Finally, we define an operation to update that data. As such we have an (IntraFieldOperation(), 'block_path') tuple. We can have as many as these as we like in our operations_and_block_paths, but for now we'll look at a single one for our rename operation.

In this case the block that we are operating on is stream1, the parent of the block being renamed (refer to [](rename_stream_children_operation) - for rename and remove operations we always operate on the parent block). In that case our block path will be stream1. Next we need a function that will update our data. For this, the wagtail.blocks.operations module has a set of commonly used intra-field operations available (and it is possible to write custom operations). Since this is a rename operation that operates on a StreamField, we will use wagtail.blocks.operations.RenameStreamChildrenOperation which accepts two arguments as the old block name and the new block name. As such our operation and block path tuple will look like this:

(RenameStreamChildrenOperation(old_name="field1", new_name="block1"), "stream1")

And our final code will be:

from django.db import migrations

from wagtail.blocks.migrations.migrate_operation import MigrateStreamData
from wagtail.blocks.migrations.operations import RenameStreamChildrenOperation

class Migration(migrations.Migration):

    dependencies = [
        ...
    ]

    operations = [
        MigrateStreamData(
            app_name="blog",
            model_name="BlogPage",
            field_name="content",
            operations_and_block_paths=[
                (RenameStreamChildrenOperation(old_name="field1", new_name="block1"), "stream1"),
            ]
        ),
    ]

(using_streamfield_migration_block_paths)=

Using operations and block paths properly

The MigrateStreamData class takes a list of operations and corresponding block paths as a parameter operations_and_block_paths. Each operation in the list will be applied to all blocks that match the corresponding block path.

operations_and_block_paths=[
    (operation1, block_path1),
    (operation2, block_path2),
    ...
]

Block path

The block path is a '.'-separated list of names of the block types from the top level StreamBlock (the container of all the blocks in the StreamField) to the nested block type which will be matched and passed to the operation.

If we want to operate directly on the top level `StreamBlock`, then the block path must be
an empty string `""`.

For example, if our stream definition looks like this:

class MyDeepNestedBlock(StreamBlock):
    foo = CharBlock()
    date = DateBlock()

class MyNestedBlock(StreamBlock):
    char1 = CharBlock()
    deepnested1 = MyDeepNestedBlock()

class MyStreamBlock(StreamBlock):
    field1 = CharBlock()
    nested1 = MyNestedBlock()

class MyPage(Page):
    content = StreamField(MyStreamBlock)

If we want to match all "field1" blocks, our block path will be "field1":

[
    { "type": "field1", ... }, # this is matched
    { "type": "field1", ... }, # this is matched
    { "type": "nested1", "value": [...] },
    { "type": "nested1", "value": [...] },
    ...
]

If we want to match all "deepnested1" blocks, which are a direct child of "nested1", our block path will be "nested1.deepnested1":

[
    { "type": "field1", ... },
    { "type": "field1", ... },
    { "type": "nested1", "value": [
        { "type": "char1", ... },
        { "type": "deepnested1", ... }, # This is matched
        { "type": "deepnested1", ... }, # This is matched
        ...
    ] },
    { "type": "nested1", "value": [
        { "type": "char1", ... },
        { "type": "deepnested1", ... }, # This is matched
        ...
    ] },
    ...
]

When the path contains a ListBlock child, 'item' must be added to the block path as the name of said child. For example, if we consider the following stream definition:

class MyStructBlock(StructBlock):
    char1 = CharBlock()
    char2 = CharBlock()

class MyStreamBlock(StreamBlock):
    list1 = ListBlock(MyStructBlock())

Then if we want to match "char1", which is a child of the StructBlock which is the direct list child, we have to use block_path_str="list1.item.char1" instead of block_path_str="list1.char1". We can also match the ListBlock child with block_path_str="list1.item".

Rename and remove operations

The following operations are available for renaming and removing blocks.

• RenameStreamChildrenOperation
• RenameStructChildrenOperation
• RemoveStreamChildrenOperation
• RemoveStructChildrenOperation

Note that all of these operations operate on the value of the parent block of the block which must be removed or renamed. Hence make sure that the block path you are passing points to the parent block when using these operations (see the example in basic usage).

Alter block structure operations

The following operations allow you to alter the structure of blocks in certain ways.

• [](stream_children_to_list_block_operation): operates on the value of a StreamBlock. Combines all child blocks of type block_name as children of a single ListBLock which is a child of the parent StreamBlock.
• [](stream_children_to_stream_block_operation): operates on the value of a StreamBlock. Note that block_names here is a list of block types and not a single block type unlike block_name in the previous operation. Combines each child block of a type in block_names as children of a single StreamBlock which is a child of the parent StreamBlock.
• [](stream_children_to_struct_block_operation): moves each StreamBlock child of the given type inside a new StructBlock

A new StructBlock will be created as a child of the parent StreamBlock for each child block of the given type, and then that child block will be moved from the parent StreamBlock's children inside the new StructBlock as a child of that StructBlock.

For example, consider the following StreamField definition:

mystream = StreamField([("char1", CharBlock()) ...], ...)

Then the stream data would look like the following:

[
    { "type": "char1", "value": "Value1", ... },
    { "type": "char1", "value": "Value2", ... },
    ...
]

And if we define the operation like this:

StreamChildrenToStructBlockOperation("char1", "struct1")

Our altered stream data would look like this:

[
    ...
    { "type": "struct1", "value": { "char1": "Value1" } },
    { "type": "struct1", "value": { "char1": "Value2" } },
    ...
]

Block ids are not preserved here since the new blocks are structurally different than the previous blocks.

Other operations

• [](alter_block_value_operation)

(custom_streamfield_migration_operations)=

Making custom operations

Basic usage

While this package comes with a set of operations for common use cases, there may be many instances where you need to define your own operation for mapping data. Making a custom operation is fairly straightforward. All you need to do is extend the BaseBlockOperation class and define the required methods,

• apply
  This applies the actual changes to the existing block value and returns the new block value.
• operation_name_fragment
  (@property) Returns a name to be used for generating migration names.

(NOTE: BaseBlockOperation inherits from abc.ABC, so all of the required methods mentioned above have to be defined on any class inheriting from it.)

For example, if we want to truncate the string in a CharBlock to a given length,

from wagtail.blocks.migrations.operations import BaseBlockOperation

class MyBlockOperation(BaseBlockOperation):
    def __init__(self, length):
        super().__init__()
        # we will need to keep the length as an attribute of the operation
        self.length = length

    def apply(self, block_value):
        # block value is the string value of the CharBlock
        new_block_value = block_value[:self.length]
        return new_block_value


    @property
    def operation_name_fragment(self):
        return "truncate_{}".format(self.length)

block_value

Note that depending on the type of block we're dealing with, the block_value which is passed to apply may take different structures.

For non-structural blocks, the value of the block will be passed directly. For example, if we're dealing with a CharBlock, it will be a string value.

The value passed to apply when the matched block is a StreamBlock would look like this,

[
    { "type": "...", "value": "...", "id": "..." },
    { "type": "...", "value": "...", "id": "..." },
    ...
]

The value passed to apply when the matched block is a StructBlock would look like this,

{
    "type1": "...",
    "type2": "...",
    ...
}

The value passed to apply when the matched block is a ListBlock would look like this,

[
    { "type": "item", "value": "...", "id": "..." },
    { "type": "item", "value": "...", "id": "..." },
    ...
]

Making structural changes

When making changes involving the structure of blocks (changing the block type for example), it may be necessary to operate on the block value of the parent block instead of the block to which the change is made, since only the value of a block is changed by the apply operation.

Take a look at the implementation of RenameStreamChildrenOperation for an example.

Old list format

Prior to Wagtail version 2.16, ListBlock children were saved as just a normal Python list of values. However, for newer versions of Wagtail, list block children are saved as ListValues. When handling raw data, the changes would look like the following:

Old format

[
    value1,
    value2,
    ...
]

New format

[
    { "type": "item", "id": "...", "value": value1 },
    { "type": "item", "id": "...", "value": value2 },
    ...
]

When defining an operation that operates on a ListBlock value, in case you have old data which is still in the old format, it is possible to use wagtail.blocks.migrations.utils.formatted_list_child_generator to obtain the children in the new format like so:

    def apply(self, block_value):
        for child_block in formatted_list_child_generator(list_block_value):
            ...