我正在尝试在SQLAlchemy中为物化视图自动生成迁移，唯一不起作用的是检测修订之间的更改。更具体地说，即使模型没有改变，总是也会产生变化。
我认为问题是，我可能只是在这里使用错误的方法，是schema.compile(compile_kwargs={'literal_binds': True})生成的SQL与inspector.get_view_definition(name)返回的SQL之间的区别。
备份，这是模型定义的样子，只是一个简单的邻接列表树。使用物化视图的目的是自动生成节点的深度和路径，以便可以查询它：
from app.extensions import db # Flask-SQLAlchemy
@db.slugify('name')
class Node(db.Model):
id = db.Column(db.BigInteger, primary_key=True)
name = db.Column(db.String, index=True)
slug = db.Column(db.String, index=True)
parent_id = db.foreign_key('Node', nullable=True)
parent = db.relationship('Node', back_populates='children', remote_side=id)
children = db.relationship('Node', back_populates='parent')
mv = db.relationship('NodeMV', uselist=False, foreign_keys='NodeMV.id',
primaryjoin='Node.id == NodeMV.id')
depth = db.association_proxy('mv', 'depth')
path = db.association_proxy('mv', 'path')
_cte = (db.select([Node.id.label('id'),
literal(0).label('depth'),
literal('/').label('path')])
.where(Node.parent_id is None)
.cte(name='nodes_cte', recursive=True))
_union = _cte.union_all(
db.select([
Node.id.label('id'),
label('depth', _cte.c.depth + 1),
label('path', case([(_cte.c.depth == 0, _cte.c.path + Node.slug)],
else_=_cte.c.path + '/' + Node.slug))])
.select_from(db.join(_cte, Node, _cte.c.id == Node.parent_id))
)
class NodeMV(db.Model):
__tablename__ = 'node_mv'
__table__ = create_materialized_view('node_mv', db.select([_union]))
db.Index('ix_node_mv_id', NodeMV.id, unique=True)
create_materialized_view工厂方法的位置如下：
def create_materialized_view(name, selectable, metadata=db.metadata):
# must use a temporary metadata here so that SQLAlchemy doesn't detect the
# table as "standalone". (it will still use the correct metadata once
# attached to the __table__ attribute of the declarative base model)
table = db.Table(name, db.MetaData())
for col in selectable.c:
table.append_column(
db.Column(col.name, col.type, primary_key=col.primary_key))
if not any([col.primary_key for col in selectable.c]):
table.append_constraint(
db.PrimaryKeyConstraint(*[col.name for col in selectable.c]))
# to support using db.drop_all()
db.event.listen(metadata, 'before_drop',
db.DDL(f'DROP MATERIALIZED VIEW IF EXISTS {name}'))
# to support auto-generated migrations
metadata.info.setdefault('materialized_views', set()).add(
(name, selectable))
metadata.info.setdefault('materialized_tables', {})[name] = table
return table
检测更改的迁移比较器：
class ReplaceableSql:
def __init__(self, name, create_sql, drop_sql, indexes, **kwargs):
self.name = name
self.create_sql = create_sql
self.drop_sql = drop_sql
self.indexes = indexes
self.kwargs = kwargs
def __eq__(self, other):
if not isinstance(other, ReplaceableSql):
return False
return self.__hash__() == other.__hash__()
def __hash__(self):
key = (self.name, re.sub(r'\s+', ' ', self.create_sql), self.indexes)
return hash(key)
def create_replaceable_sql(name, schema, metadata):
query = (schema if isinstance(schema, str)
else schema.compile(compile_kwargs={'literal_binds': True}))
indexes = [(index.name, [c.name for c in index.columns], index.unique)
for index in metadata.info['materialized_tables'][name].indexes]
return ReplaceableSql(name,
f'CREATE MATERIALIZED VIEW {name} AS {query}',
f'DROP MATERIALIZED VIEW IF EXISTS {name}',
indexes)
@comparators.dispatch_for('schema')
def compare_views(autogen_context, upgrade_ops, schemas):
inspector = autogen_context.inspector
metadata = autogen_context.metadata
prev_revision = autogen_context.migration_context.get_current_revision()
# existing views
views = set()
view_names = set()
for schema in schemas:
for name in inspector.get_view_names(include='materialized', schema=schema):
views.add(create_replaceable_sql(
name, inspector.get_view_definition(name, schema=schema), metadata))
view_names.add(name)
# current metadata views
metadata_views = set()
metadata_view_names = set()
for name, selectable in metadata.info.setdefault('materialized_views', set()):
metadata_views.add(create_replaceable_sql(name, selectable, metadata))
metadata_view_names.add(name)
creates = metadata_views.difference(views)
drops = views.difference(metadata_views)
upgrades = view_names.intersection(metadata_view_names)
for replaceable_sql in creates:
name = replaceable_sql.name
if name in upgrades:
replaceable_sql.kwargs['prev'] = f'{prev_revision}.{name}'
upgrade_ops.ops.append(CreateOp(replaceable_sql))
for replaceable_sql in drops:
if replaceable_sql.name in upgrades:
continue
upgrade_ops.ops.append(DropOp(replaceable_sql))
检查与生成的SQL之间的差异很小（忽略空格），但它们足以甩掉字符串比较：
inspector.get_view_definition（表名）：
WITH RECURSIVE nodes_cte(id, depth, path) AS (
SELECT
node.id,
0 AS depth,
'/'::text AS path
FROM node
WHERE false
UNION ALL
SELECT
node.id,
(nodes_cte_1.depth + 1) AS depth,
CASE
WHEN (nodes_cte_1.depth = 0) THEN (nodes_cte_1.path || (node.slug)::text)
ELSE ((nodes_cte_1.path || '/'::text) || (node.slug)::text)
END AS path
FROM (nodes_cte nodes_cte_1
JOIN node ON ((nodes_cte_1.id = node.parent_id)))
)
SELECT nodes_cte.id, nodes_cte.depth, nodes_cte.path FROM nodes_cte;
schema.compile（compile_kwargs = {'literal_binds'：True}）：
WITH RECURSIVE nodes_cte(id, depth, path) AS (
SELECT
node.id AS id,
0 AS depth,
'/' AS path
FROM node
WHERE false
UNION ALL
SELECT
node.id AS id,
nodes_cte.depth + 1 AS depth,
CASE
WHEN (nodes_cte.depth = 0) THEN nodes_cte.path || node.slug
ELSE nodes_cte.path || '/' || node.slug
END AS path
FROM nodes_cte
JOIN node ON nodes_cte.id = node.parent_id
)
SELECT nodes_cte.id, nodes_cte.depth, nodes_cte.path FROM nodes_cte
所以我想问题是，我应该采取什么样的正确方法？ （也可能使用物化视图是解决我问题的错误方法 - 我确实尝试使用@hybrid_property / @prop_name.expression的东西，但我无法弄清楚如何使其与递归一起工作数据的性质。）
提前致谢！