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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Question: Field selection and error handling in a batch loader?","articleBody":"I have two unrelated questions:\r\n\r\n### 1. Error handling\r\n\r\nNeither `org.dataloader.BatchLoader::load` nor `org.dataloader.BatchLoaderWithContext::load` allow one to throw an exception while attempting to fetch some data from some source, because the interface does not declare that an exception is thrown:\r\n\r\n```java\r\n    CompletionStage\u003cList\u003cV\u003e\u003e load(List\u003cK\u003e keys);\r\n```\r\n\r\nWhat is the correct way to handle an error, when, for example, my repository blows up while trying to load a list of entities by the provided keys? Similarly, what if some external HTTP request to fetch a list of entities fails in an unrecoverable way? It seems wrong in these cases to return an empty list; I would prefer to throw an exception and cause the entire GraphQL call stack to unwind instead of returning partial data.\r\n\r\n### 2. Field selection\r\n\r\nThe `BatchLoaderEnvironment` passed to a `BatchLoaderWithContext::load` call does not allow me to inspect the request for the selected fields that caused loading of some entities to occur. Is there a way to do this? I prefer to select only the columns absolutely necessary from the database/API/etc. to optimize performance.\r\n\r\nI can imagine that it may be difficult to traverse all nodes on the query to aggregate the minimum acceptable selection of fields from a given type, but it doesn't seem impossible. What I'm hoping is possible is something like this:\r\n\r\nGiven a query like this:\r\n```\r\nquery {\r\n  foo {\r\n    bar {\r\n      fieldA\r\n      fieldB\r\n      fieldC\r\n    }\r\n  }\r\n  bar {\r\n    fieldA\r\n    fieldD\r\n  }\r\n}\r\n```\r\n\r\n...that I can be able to then only select fields A, B, C, and D, instead of a `SELECT *` (assuming a relational database is the backing store).\r\n\r\nThanks for your help!","author":{"url":"https://github.com/timrourke","@type":"Person","name":"timrourke"},"datePublished":"2019-09-23T22:26:53.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":1},"url":"https://github.com/53/java-dataloader/issues/53"}