René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Schematics: Unit testing a rule that uses mergeWith/apply fails due to missing context","articleBody":"## Addendum\r\n\r\nIn creating the test case I've realised that I'm not passing a `partialContext` to `callRule`, so it is definitely missing context, however, it is not at all clear _what_ context is necessary for `mergeWith` to succeed and the error does not indicate that context is actually missing.\r\n\r\nHappy for this to be marked as a question instead. Guidance on _how_ to provide the context and _what_ context to provide would be amazingly useful.\r\n\r\n# 🐞 Bug report\r\n\r\n### Description\r\n\r\nUsing `callRule` to unit test rules within a schematic fails when it comes to rules which return `mergeWith`\r\n\r\n## 🔬 Minimal Reproduction\r\n\r\nSee: https://github.com/jdpearce/schematics-apply-problem\r\n\r\nGiven a rule which applies template files to the tree, this unit test will fail with `TypeError: Cannot read property 'path' of undefined`:\r\n\r\n```ts\r\ndescribe('applyTemplateFiles Rule', () =\u003e {\r\n  let tree: UnitTestTree;\r\n\r\n  beforeEach(async () =\u003e {\r\n    tree = new UnitTestTree(Tree.empty());\r\n  });\r\n\r\n  it('should apply the files to the tree', async () =\u003e {\r\n    const schema = {};\r\n\r\n    tree = (await callRule(applyTemplateFiles(schema), tree)) as UnitTestTree;\r\n\r\n    expect(tree.files).toEqual(jasmine.arrayContaining(['/libs/destination/test.txt']));\r\n  });\r\n});\r\n```\r\n\r\n## 🔥 Exception or Error\r\n\r\n```bash\r\nFailures:\r\n1) applyTemplateFiles Rule should apply the files to the tree\r\n  Message:\r\n    TypeError: Cannot read property 'path' of undefined\r\n  Stack:\r\n        at \u003cJasmine\u003e\r\n        at ./apply-problem/node_modules/@angular-devkit/schematics/tools/file-system-engine-host-base.js:216:96\r\n        at ./tmp/apply-problem/node_modules/@angular-devkit/schematics/src/rules/url.js:13:73\r\n        at Object.callSource (./tmp/apply-problem/node_modules/@angular-devkit/schematics/src/rules/call.js:55:20)\r\n        at ./tmp/apply-problem/node_modules/@angular-devkit/schematics/src/rules/base.js:45:60\r\n        at Object.callSource (./tmp/apply-problem/node_modules/@angular-devkit/schematics/src/rules/call.js:55:20)\r\n        at ./tmp/apply-problem/node_modules/@angular-devkit/schematics/src/rules/base.js:53:23\r\n        at MergeMapSubscriber.project (./tmp/apply-problem/node_modules/@angular-devkit/schematics/src/rules/call.js:74:24)\r\n        at MergeMapSubscriber._tryNext (./tmp/apply-problem/node_modules/rxjs/internal/operators/mergeMap.js:69:27)\r\n        at MergeMapSubscriber._next (./tmp/apply-problem/node_modules/rxjs/internal/operators/mergeMap.js:59:18)\r\n        at MergeMapSubscriber.Subscriber.next (./tmp/apply-problem/node_modules/rxjs/internal/Subscriber.js:66:18)\r\n\r\n2 specs, 1 failure\r\n```\r\n\r\n## 🌍 Your Environment\r\n\r\n    \"@angular-devkit/core\": \"^9.0.6\",\r\n    \"@angular-devkit/schematics\": \"^9.0.6\",\r\n    \"@types/jasmine\": \"^3.3.9\",\r\n    \"@types/node\": \"^8.0.31\",\r\n    \"jasmine\": \"^3.3.1\",\r\n    \"typescript\": \"~3.5.3\"\r\n","author":{"url":"https://github.com/catfireparty","@type":"Person","name":"catfireparty"},"datePublished":"2020-03-12T12:00:09.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":4},"url":"https://github.com/17205/angular-cli/issues/17205"}