[rfc] MySQL relationships

Closes https://github.com/hasura/graphql-engine-mono/issues/2096

This RFC describes the MySQL relationships feature - [rendered](https://github.com/hasura/graphql-engine-mono/blob/evie/rfc/mysql-relationships/rfcs/mysql-relationships.md)

https://github.com/hasura/graphql-engine-mono/pull/2115

GitOrigin-RevId: 9f086120e65bfee091fa4c4e75c0f00f1fe4229f

rfcs/mysql-relationships.md

@@ -0,0 +1,153 @@
+```
+---
+authors: Evie Ciobanu <evie@hasura.io>
+state: draft
+---
+```
+
+# MySQL Relationships
+
+As the name suggests, relationships are a core feature of relational
+databases. Using relationships between entities, we can query or update
+structured data as well as ensure data consistency.
+
+## Description
+
+Since relationships are such a key feature of any relational database,
+including MySQL, the Hasura engine is expected to be able to handle
+them as well.
+
+Relationships are either object or array. Object relationships represent
+a single related entity (e.g., an article is only published by one
+author), whereas array relationships may have any number of related entities
+(e.g., an author may have any number of published articles).
+
+### Success
+
+This feature allows us to track, untrack, and query both object and array
+relationships for MySQL sources.
+
+Additionally, the `TestRelationships` test suite should pass on MySQL sources
+(https://github.com/hasura/graphql-engine-mono/blob/6905d5914c8a698445c0ef03d6a8303747701e1c/server/tests-py/test_v1_queries.py#L565)
+
+The following tests should also pass for MySQL sources:
+```
+# relationships
+test_select_query_multiple_columns_arr_fkey
+test_select_query_multiple_columns_obj_fkey
+test_int_as_string_offset
+test_err_neg_offset_error
+
+# depend on relationships: nested queries
+test_nested_select_query_article_author
+test_nested_select_query_deep
+```
+
+## What
+
+This feature translates to implementing certain instance methods for
+MySQL sources, see https://github.com/hasura/graphql-engine-mono/pull/1110
+as an example.
+
+It is not currently expected that any changes would be necessary in the
+common areas of the code, other than adding the appropriate API endpoints:
+- `mysql_create_object_relationship`
+- `mysql_create_array_relationship`
+- `mysql_set_relationship_comment`
+- `mysql_rename_relationship`
+- `mysql_drop_relationship`
+
+If a need for any non-trivial changes to the common code arises (such as,
+but not limited to adding or changing method signatures in the `Backend*`
+classes, IR code, parser code, etc.), this should be brought up with the
+data sources and the IR teams to discuss options before proceeding.
+
+On top of metadata-related operations, the following features are
+implied when considering relationships:
+- selecting data using relationships
+- relationships in boolean expressions
+- relationships in permission boolean expressions
+- `_aggregate` fields for array relationships
+- ordering using relationships
+
+The above features are listed from most to least important.
+
+### Selecting data using relationships
+
+It is expected that creating a relationship creates a new field that
+is selectable. For example, getting the author of a published article
+by using`articles.author`.
+
+```
+query {
+  articles {
+    id
+    title
+	author {
+	  id
+	  name
+	}
+  }
+}
+```
+
+Relevant test: `test_nested_select_query_article_author`
+
+### Relationships in boolean expressions
+
+Filtering should work on relationships, for example, we should be able
+to filter articles by their author's name:
+
+```
+query {
+  articles (where: {author: {name: {_eq: "Sidney"}}}) {
+    id
+    title
+  }
+}
+```
+
+Relevant test: `test_nested_select_query_where_on_relationship`
+
+### Relationships in permission boolean expressions
+
+Permissions should also be usable in boolean expressions. For more
+details, please see the MySQL Permissions RFC.
+
+### Aggregate fields for array relationshipsd
+
+It is expected that each array relationship creates a `name_aggregate`
+field:
+
+```
+query {
+  autors {
+    articles_aggregate {
+	  aggregate {
+	    count
+	  }
+	}
+  }
+}
+```
+
+Relevant test: `test_author_agg_with_articles`
+
+### Ordering using relationships
+
+Nested objects should be usable in ordering clauses.
+
+```
+query {
+  articles( order_by: { author: { id: desc } } ) {
+    id
+	name
+  }
+}
+```
+
+Relevant test: `test_nested_select_query_where_on_relationship`
+
+## Effects and Interactions
+
+This feature will likely require support from the Console and CLI teams.