Ember data filter deprecation (#326)

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text/0000-ember-data-filter-deprecation.md

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+- Start Date: (fill me in with today's date, 2018-04-18)
+- RFC PR: (leave this empty)
+- Ember Issue: (leave this empty)
+
+# Ember Data Filter Deprecation
+
+## Summary
+
+Deprecate the `store.filter` API. This API was previously gated
+behind a private `ENV` variable that was enabled by the addon
+[`ember-data-filter`](https://github.com/ember-data/ember-data-filter/tree/b62c992186c00dce8cc81f1fb0cf5e2e6fee0f6b#ember-data-filter).
+
+## Motivation
+
+The `filter` API was a "memory leak by design". [Patterns exist](https://github.com/ember-data/ember-data-filter#recommended-refactor-guide)
+with no-worse ergonomics that have better performance and do not incur memory leak penalties.
+
+While the change in ergonomics for end consumers in minimal, the change to `ember-data` is substantial.
+The code for this feature required significant amounts of confusing internal plumbing to ensure that
+filters were rerun every time any form of mutation (update, addition, deletion) occurred to any record.
+
+In addition to maintenance costs, this plumbing negatively affects the performance of all `RecordArray`s,
+ and slow any operations that count as mutations (such as pushing new records into the store).
+
+By removing this feature, we significantly simplify and streamline the core of `Ember Data`.
+
+## Detailed design
+
+We will provide 3 new deprecations with links to a [guide on how to refactor](https://github.com/ember-data/ember-data-filter#recommended-refactor-guide).
+These deprecations will target `3.5`, meaning that the `ember-data-filter` addon will continue to
+work and be supported through the release of ember-data `3.4`.
+
+**Deprecation: ember-data-filter:filter**
+
+Deprecate the primary case (`store.filter('posts', filterFn)`).
+Instead, users can combine `store.peekAll` with a computed property.
+
+**Deprecation: ember-data-filter:query-for-filter**
+
+This deprecation is specific to folks providing a `query` to be requested the
+first time a filter is run. To do this better, users can separate their usage
+of `filter` from their usage of `query`.
+
+**Deprecation: ember-data-filter:empty-filter**
+
+In the case that users were creating a `filter` with no method for filtering by,
+a deprecation is printed letting them know that the easiest path forward is to
+use `peekAll`, which would return the same record result set.
+
+## How we teach this
+
+The `filter` API is rarely used, having been discouraged for many years. A simple post
+ alerting users to it's deprecation should be sufficient. The refactoring guide is
+ sufficiently simple that teaching folks a better way should not be much of a hurdle.
+
+## Drawbacks
+
+Minor churn for folks that did use this API; however, the end result will improve the
+performance of apps using filters more so than anyone else.
+
+## Alternatives
+
+There's been some talk of an API for local querying; however, said alternative RFC
+ would only result in deprecating this API as well.
+
+## Unresolved questions
+
+None