[Bug]: Cannot return null for non-nullable field.

[yorek]

What happened?

If, in a One-to-Many or Many-to-One relationship, there is no value for the "one" side of the relationship, an error is returned, instead than returning a null value. As a result, this make impossible to represent correctly One-to-One relationship. Take this example, using the "School Database" available here: https://learn.microsoft.com/en-us/ef/ef6/resources/school-database

There is a one-to-one relationship between Course and OnlineCourse as OnlineCourse is a "specialization" of Course. Same goes between Course and OnsiteCourse and This is the following DAB config:

{
  "$schema": "https://github.com/Azure/data-api-builder/releases/download/v0.8.52/dab.draft.schema.json",
  "data-source": {
    "database-type": "mssql",
    "connection-string": "@env('MSSQL')",
    "options": {
      "set-session-context": false
    }
  },
  "runtime": {
    "rest": {
      "enabled": true,
      "path": "/api"
    },
    "graphql": {
      "enabled": true,
      "path": "/graphql",
      "allow-introspection": true
    },
    "host": {
      "cors": {
        "origins": [],
        "allow-credentials": false
      },
      "authentication": {
        "provider": "StaticWebApps"
      },
      "mode": "development"
    }
  },
  "entities": {
    "Course": {
      "source": {
        "object": "dbo.Course",
        "type": "table"
      },
      "graphql": {
        "enabled": true,
        "type": {
          "singular": "Course",
          "plural": "Courses"
        }
      },
      "rest": {
        "path": "/courses",
        "enabled": true
      },
      "permissions": [
        {
          "role": "anonymous",
          "actions": [
            {
              "action": "*"
            }
          ]
        }
      ],
      "relationships": {
        "OnlineCourse": {
          "cardinality": "one",
          "target.entity": "OnlineCourse"
        },
        "OnsiteCourse": {
          "cardinality": "one",
          "target.entity": "OnsiteCourse"
        }
      }
    },
    "OnlineCourse": {
      "source": {
        "object": "dbo.OnlineCourse",
        "type": "table"
      },
      "graphql": {
        "enabled": true,
        "type": {
          "singular": "OnlineCourse",
          "plural": "OnlineCourses"
        }
      },
      "rest": {
        "path": "/online-courses",
        "enabled": true
      },
      "permissions": [
        {
          "role": "anonymous",
          "actions": [
            {
              "action": "*"
            }
          ]
        }
      ],
      "relationships": {
        "Course": {
          "cardinality": "one",
          "target.entity": "Course"
        }
      }
    },
    "OnsiteCourse": {
      "source": {
        "object": "dbo.OnsiteCourse",
        "type": "table"
      },
      "graphql": {
        "enabled": true,
        "type": {
          "singular": "OnsiteCourse",
          "plural": "OnsiteCourses"
        }
      },
      "rest": {
        "path": "/onsite-courses",
        "enabled": true
      },
      "permissions": [
        {
          "role": "anonymous",
          "actions": [
            {
              "action": "*"
            }
          ]
        }
      ],
      "relationships": {
        "Course": {
          "cardinality": "one",
          "target.entity": "Course"
        }
      }
    }
  }
}

if I run the following GraphQL query:

query {
  courses {
    items {
      Title
      OnlineCourse {   
          URL
      }
      OnsiteCourse {
          Location
          Days
      }
    }
  }
}

I get the following error:

{
  "errors": [
    {
      "message": "Cannot return null for non-nullable field.",
      "locations": [
        {
          "line": 5,
          "column": 7
        }
      ],
      "path": [
        "courses",
        "items",
        0,
        "OnlineCourse"
      ],
      "extensions": {
        "code": "HC0018"
      }
    }
  ]
}

As for not all courses the field returned by the referenced entities will be available.

Version

Microsoft.DataApiBuilder 0.8.52+c69925060e1942d28515b9c4b89ea24832da0c7c

What database are you using?

Azure SQL

What hosting model are you using?

Local (including CLI)

Which API approach are you accessing DAB through?

GraphQL

Relevant log output

Microsoft.AspNetCore.Routing.EndpointMiddleware[0]
      Executing endpoint 'Hot Chocolate GraphQL Pipeline'
dbug: Azure.DataApiBuilder.Core.AuthenticationHelpers.ClientRoleHeaderAuthenticationMiddleware[0]
      1c429351-7b86-4eb2-9298-2e56e41dafe9: Request authentication state: Anonymous.
dbug: Azure.DataApiBuilder.Core.AuthenticationHelpers.ClientRoleHeaderAuthenticationMiddleware[0]
      1c429351-7b86-4eb2-9298-2e56e41dafe9: The request will be executed in the context of Anonymous role
dbug: Azure.DataApiBuilder.Core.Resolvers.IQueryExecutor[0]
      1c429351-7b86-4eb2-9298-2e56e41dafe9: Executing query:
SELECT TOP 100 [table0].[Title] AS [Title], JSON_QUERY ([table1_subq].[data]) AS [OnlineCourse], JSON_QUERY ([table3_subq].[data]) AS [OnsiteCourse] FROM [dbo].[Course] AS [table0] OUTER APPLY (SELECT TOP 1 [table1].[URL] AS [URL] FROM [dbo].[OnlineCourse] AS [table1] WHERE [table1].[CourseID] = [table0].[CourseID] ORDER BY [table1].[CourseID] ASC FOR JSON PATH, INCLUDE_NULL_VALUES,WITHOUT_ARRAY_WRAPPER) AS [table1_subq]([data]) OUTER APPLY (SELECT TOP 1 [table3].[Location] AS [Location], [table3].[Days] AS [Days] FROM [dbo].[OnsiteCourse] AS [table3] WHERE [table3].[CourseID] = [table0].[CourseID] ORDER BY [table3].[CourseID] ASC FOR JSON PATH, INCLUDE_NULL_VALUES,WITHOUT_ARRAY_WRAPPER) AS [table3_subq]([data]) WHERE 1 = 1 ORDER BY [table0].[CourseID] ASC FOR JSON PATH, INCLUDE_NULL_VALUES
info: Microsoft.AspNetCore.Routing.EndpointMiddleware[1]
      Executed endpoint 'Hot Chocolate GraphQL Pipeline'
info: Microsoft.AspNetCore.Hosting.Diagnostics[2]
      Request finished HTTP/2 POST https://localhost:5001/graphql application/json 212 - 500 - application/json;+charset=utf-8 5.6927ms

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