add adr for 'object contains'

adr/2024-02-object-contains.md

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+# `contains` and JSON Objects
+
+* Status: Proposed, accepted, reconsidered, and ultimately reverted.
+* Deciders: @gregsdennis, @jdesrosiers, @handrews, @awwright, @karenetheridge, @relequestual (with input from a couple non-core members)
+* Date: 2023-11-14 (documented 2024-02-09)
+
+Technical Story:
+
+- Original proposal [#1077](https://github.com/json-schema-org/json-schema-spec/issues/1077) and [PR](https://github.com/json-schema-org/json-schema-spec/pull/1092)
+- Reversion discussion [#1358](https://github.com/json-schema-org/json-schema-spec/issues/1358) and [PR](https://github.com/json-schema-org/json-schema-spec/pull/1452)
+
+## Context and Problem Statement
+
+[2021-02]
+The original proposal was for `contains` to apply to objects as well as arrays since there was no functionality to do so.
+The discussion covered the options of modifying `contains` or introducing a new `objectContains` (or similar) keyword set (also needs separate min/max).
+The decision was voted on and modifying `contains` won.
+
+[2021-06]
+A change was applied.
+
+[2022-12]
+With the team shifting focus to stability between spec releases, the question was raised again with the argument that allowing `contains` to apply to objects is a breaking change.
+It was conceded that the better approach would be to retain `contains` only for arrays and introduce a new keyword set to apply to objects.
+
+[2023-11]
+The change was applied (reverted to previous behavior).
+
+## Decision Drivers <!-- optional -->
+
+* The original decision to allow `contains` to apply to objects was driven by the fact that no such functionality existed.
+* The decision to revert was driven by a desire to not break current usages of `contains`.
+
+## Considered Options
+
+* `contains` could be modified to apply to objects.
+* a new keyword set (e.g. `objectContains` along with associated min/max) could be added.
+
+## Decision Outcome
+
+Ultimately, `contains` will continue to apply to only arrays.
+New keywords will need to be introduced to apply to objects.
+(Such a proposal has not yet been made.)
+
+### Positive Consequences <!-- optional -->
+
+* Schemas which currently use `contains` without a `type: array` specifier will not suddenly start applying to objects also.
+
+### Negative Consequences <!-- optional -->
+
+* The functionality of `contains` as applied to objects is still unsupported.
+
+## Pros and Cons of the Options <!-- optional -->
+
+### Change `contains`
+
+(Example provided recently by a user in [Slack](https://json-schema.slack.com/archives/C5CF75URH/p1707258032879409))
+
+The requirement is that an object may contain any number of properties, but one and only one of them must contain an object with a `title` property.
+
+✔️ valid
+```json
+{
+  "foo": { "title": "a title" },
+  "bar": { "baz": 42 }
+}
+```
+
+❌ invalid
+```json
+{
+  "foo": { "quux": false },
+  "bar": { "baz": 42 }
+}
+```
+
+❌ invalid
+```json
+{
+  "foo": { "title": "a title" },
+  "bar": { "title": "a title" }
+}
+```
+
+Currently, this is impossible since there is no way to conditionally count property values.
+However, with `contains` applying to objects, the following is possible:
+
+```json
+{
+  "type": "object",
+  "contains": {
+    "type": "object",
+    "required": ["title"]
+  },
+  "minContains": 1,
+  "maxContains": 1
+}
+```
+
+* Good, because it provides functionality that previously did not exist
+* Bad, because is can potentially break some schemas
+* … <!-- numbers of pros and cons can vary -->
+
+### New keywords
+
+Same examples as [changing `contains`](#change-contains), except we use new keywords instead.
+
+The schema would be something like this instead:
+
+```json
+{
+  "type": "object",
+  "objectContains": {
+    "type": "object",
+    "required": ["title"]
+  },
+  "objectMinContains": 1,
+  "objectMaxContains": 1
+}
+```
+
+* Good, because it provides functionality that previously did not exist
+* Good, because it doesn't break anyone
+* Bad, because we have to introduce three new keywords
+* … <!-- numbers of pros and cons can vary -->