add a function in our schema metadata lib for `foreignKeyCountsByOriginTable`...

add a function in our schema metadata lib for `foreignKeyCountsByOriginTable` which counts the occurrences of FK relationships by the origin table they come from.  this allows us to know if a given origin table is represented multiple times in a list of FKs.

frontend/src/lib/schema_metadata.js

@@ -418,3 +418,21 @@ export function hasLatitudeAndLongitudeColumns(columnDefs) {
     }
     return hasLatitude && hasLongitude;
 }
+
+export function foreignKeyCountsByOriginTable(fks) {
+    if (fks === null || !Array.isArray(fks)) {
+        return null;
+    }
+
+    return fks.map(function(fk) {
+        return ('origin' in fk) ? fk.origin.table.id : null;
+    }).reduce(function(prev, curr, idx, array) {
+        if (curr in prev) {
+            prev[curr]++;
+        } else {
+            prev[curr] = 1;
+        }
+
+        return prev;
+    }, {});
+}

frontend/test/unit/lib/schema_metadata.spec.js

@@ -7,7 +7,8 @@ import {
     NUMBER,
     BOOLEAN,
     LOCATION,
-    COORDINATE
+    COORDINATE,
+    foreignKeyCountsByOriginTable
 } from 'metabase/lib/schema_metadata';
 
 describe('schema_metadata', () => {
@@ -44,4 +45,16 @@ describe('schema_metadata', () => {
             expect(getFieldType({ base_type: 'DERP DERP DERP' })).toEqual(undefined)
         });
     });
+
+    describe('foreignKeyCountsByOriginTable', () => {
+        it('should work with null input', () => {
+            expect(foreignKeyCountsByOriginTable(null)).toEqual(null)
+        });
+        it('should require an array as input', () => {
+            expect(foreignKeyCountsByOriginTable({})).toEqual(null)
+        });
+        it('should count occurrences by origin.table.id', () => {
+            expect(foreignKeyCountsByOriginTable([{ origin: {table: {id: 123}} }, { origin: {table: {id: 123}} }, { origin: {table: {id: 123}} }, { origin: {table: {id: 456}} }])).toEqual({123: 3, 456: 1})
+        });
+    });
 });