Describe the bug

Can't search by ID in relation in with postgres database

Steps to reproduce the behavior

1. Create a new project yarn create strapi-app my-project --quickstart
2. Change database to postgres
3. Run the application yarn develop
4. Create a dummy model with Content-Type Builder that has a relation with User example Documents

{
  "kind": "collectionType",
  "collectionName": "documents",
  "info": {
    "name": "Documents"
  },
  "options": {
    "increments": true,
    "timestamps": true
  },
  "attributes": {
    "Name": {
      "type": "string"
    },
    "user": {
      "plugin": "users-permissions",
      "model": "user",
      "via": "documents"
    }
  }
}

5. Click "Configure the view" and for "User" select "id" field as "Entry Title"

6. Create at least 2 users in the panel
7. Try to create a new model, in User field when you try to find users by id field error occurs and you cannot find this

Once you start to autocomplete search you get admin error, which is trying to find that model by given type input
calling Admin API with /content-manager/explorer/application::XXX.XXX?id_contains=23&_limit=20&_start=0

Console throws an error

error error: function lower(integer) does not exist
    at Parser.parseErrorMessage (/Users/qunabu/Desktop/localhost/quickstart-strapi/node_modules/pg-protocol/dist/parser.js:278:15)
    at Parser.handlePacket (/Users/qunabu/Desktop/localhost/quickstart-strapi/node_modules/pg-protocol/dist/parser.js:126:29)
    at Parser.parse (/Users/qunabu/Desktop/localhost/quickstart-strapi/node_modules/pg-protocol/dist/parser.js:39:38)
    at Socket.<anonymous> (/Users/qunabu/Desktop/localhost/quickstart-strapi/node_modules/pg-protocol/dist/index.js:8:42)
    at Socket.emit (events.js:311:20)
    at addChunk (_stream_readable.js:294:12)
    at readableAddChunk (_stream_readable.js:275:11)
    at Socket.Readable.push (_stream_readable.js:209:10)
    at TCP.onStreamRead (internal/stream_base_commons.js:186:23)

Expected behavior

No error, should be able to find model by id

Code snippets

This can be fixed by extending postgres and adding lower(integer) function definition to database by adding the following code to config/functions/bootstrap.js

  try {
    if (strapi.connections.default.client.config.client === "pg") {
      await strapi.connections.default.raw(
        `CREATE OR REPLACE FUNCTION lower(id INTEGER)
    RETURNS TEXT AS $$
BEGIN
    -- casts integer to text
    RETURN lower(cast(id as TEXT));
END; $$
LANGUAGE plpgsql`
      );
    }
  } catch (err) {
    console.log("error", err);
  }

System

• Node.js version: v12.16.1
• NPM version:6.13.4
• Strapi version: 3.1.4
• Database: postgres
• Operating system: mac os 10.15.6

Additional context
Error is because you cannot run lower function on non-text fields in postgres query. They should be cast to text before the query. The framework is trying to the the following query select distinct "XXX".* from "XXX" where LOWER("XXX"."id") LIKE LOWER(?) limit ?(where XXX is your model name) which is invalid in postgres