Update constraints.md (#6053)

website/docs/reference/resource-properties/constraints.md

@@ -23,6 +23,14 @@ The structure of a constraint is:
 - `name` (optional): Human-friendly name for this constraint. Supported by some data platforms.
 - `columns` (model-level only): List of column names to apply the constraint over
 
+<VersionBlock lastVersion="1.8">
+
+When using `foreign_key`, you need to specify the referenced table's schema manually. Use `{{ target.schema }}` in the `expression` field to automatically pass the schema used by the target environment. Note that later versions of dbt will have more efficient ways of handling this. 
+
+For example: `expression: "{{ target.schema }}.customers(customer_id)"`
+
+</VersionBlock>
+
 <File name='models/schema.yml'>
 
 ```yml
@@ -62,6 +70,8 @@ models:
 
 </File>
 
+
+
 ## Platform-specific support
 
 In transactional databases, it is possible to define "constraints" on the allowed values of certain columns, stricter than just the data type of those values. For example, Postgres supports and enforces all the constraints in the ANSI SQL standard (`not null`, `unique`, `primary key`, `foreign key`), plus a flexible row-level `check` constraint that evaluates to a boolean expression.