Query system tables for multiple cascade paths - sql-server

Question

I use Entity Framework to generate my tables from my model, and every now and again I get a problem with multiple cascade paths. e.g:
Introducing FOREIGN KEY constraint
'FK_dbo.SurveyorSurveys_dbo.Surveys_Survey_ID' on table
'SurveyorSurveys' may cause cycles or multiple cascade paths.
I find it quite time consuming working out where the cascade paths are to this new table, so that I can remove one of the cascade deletes. I think it would be really useful if I could run a query on the system tables that showed me the full paths that were involved. Has anyone worked out how to do that?
For example I have this sql:
SELECT
ro.name referenced_object,
po.name parent_object,
fk.name foreign_key
FROM sys.foreign_keys fk
INNER JOIN sys.all_objects po
ON fk.parent_object_id = po.object_id
INNER JOIN sys.all_objects ro
ON fk.referenced_object_id = ro.object_id
WHERE delete_referential_action_desc = 'CASCADE'
and (po.name = 'Surveys' or po.name = 'Surveyors'')
ORDER BY ro.name
But this only takes me up one level in the hierarchy, it might be better to remove a cascade delete at a higher level, so I end up adjusting and re-running the sql until I work out the full path

Answer 1

This is typically something to do with a recursive query.
First a few words to clarify the column names in sys.all_objects:
parent_object_id refers to the object owning the foreign key, so it's not the parent in the relationship, it's the child.
referenced_object_id is the parent in the relationship.
Knowing this, we can write a recursive query that collects trees of foreign key relationships by connecting the child in the previous level with the parent in the subsequent level:
WITH [Cascades] AS(
SELECT
fk.referenced_object_id AS [RootId],
fk.parent_object_id AS [ChildId],
fk.referenced_object_id AS [ParentId],
fk.name foreign_key,
1 AS [Level]
FROM sys.foreign_keys fk
WHERE delete_referential_action_desc = 'CASCADE'
UNION ALL
SELECT [Cascades].[RootId],
fk.parent_object_id,
fk.referenced_object_id,
fk.name foreign_key,
[Cascades].[Level] + 1
FROM sys.foreign_keys fk
INNER JOIN [Cascades] ON [Cascades].[ChildId] = fk.referenced_object_id
WHERE delete_referential_action_desc = 'CASCADE'
)
SELECT [root].Name AS [Root],
po.Name AS [Parent],
co.Name AS [Child],
foreign_key,
cs.[Level]
FROM [Cascades] cs
INNER JOIN sys.all_objects [root] ON cs.[RootId] = [root].object_id
INNER JOIN sys.all_objects po ON cs.ParentId = po.object_id
INNER JOIN sys.all_objects co ON cs.ChildId = co.object_id
WHERE [root].name = 'Surveys'
ORDER BY [Root], [Parent], [Level], [Child]
If I execute the query in a simple tree-level hierarchy, the output is:
Root Parent Child foreign_key Level
-----------------------------------
A A B1 FK_B1_A 1
A A B2 FK_B2_A 1
A B1 C FK_C_B1 2

Answer 2

Alternative sql based on Gert's excellent answer. This one lets me specify the leaf and shows the full cascade path on each row
WITH Cascades
AS (SELECT
fk.parent_object_id,
fk.referenced_object_id,
fk.name foreign_keys,
CAST(ro.name + ' -> ' + po.name AS nvarchar(256)) path,
1 AS level
FROM sys.foreign_keys fk
INNER JOIN sys.all_objects po
ON fk.parent_object_id = po.object_id
INNER JOIN sys.all_objects ro
ON fk.referenced_object_id = ro.object_id
WHERE delete_referential_action_desc = 'CASCADE'
-- specifying leaf instead of root:
AND po.name IN ('Surveys', 'Surveyors', 'surveyorsurveys')
UNION ALL SELECT
fk.parent_object_id,
fk.referenced_object_id,
CAST(fk.name + ' -> ' + Cascades.foreign_keys as nvarchar(128)),
CAST(ro.name + ' -> ' + cascades.path AS nvarchar(256)),
cascades.level + 1
FROM sys.foreign_keys fk
INNER JOIN sys.all_objects po
ON fk.parent_object_id = po.object_id
INNER JOIN sys.all_objects ro
ON fk.referenced_object_id = ro.object_id
INNER JOIN cascades
ON cascades.referenced_object_id = fk.parent_object_id
WHERE delete_referential_action_desc = 'CASCADE')
SELECT
path,
foreign_keys,
level
FROM Cascades
ORDER BY path