How to store likes to posts in Parse.com database

What I have done in the past is to use something like this:

1) Each document has a logical type associated with it.

2) Set up tables for metadata for each logical type. Each row can store all metadata associated with the document.

The other approach is that of key-value-modelling which can actually be ok or not depending on what you are doing with it. In this case you have a metadata table which stores all metadata, one value per record, for all documents. This works best if you find a reasonable way to aggregate the data in result sets, and if you aren't doing complex searches across multiple metadata fields.

I might suggest to reorganize schema, to make it more scalable.

1. DROP columns entityId and entityType.
2. CREATE an Objects (or Entities) table which will store public / shared information for all objects / entities. It's primary key will migrate to Companies and to Houses with 1:1 relation. For one object might exist only one company OR only one house. Also, you will have to remove AUTO_INCREMENT from Companies and Houses, because in this case they will no longer generate identifiers themselves.
3. Add foreign key to Contacts table from shared Objects table - contacts might exist for any type of object.

Structure might look like this:

You might obtain entity type by following query:

SELECT
    cnt.*,
    CASE
        WHEN NOT cmp.id IS NULL THEN 1
        WHEN NOT hs.id IS NULL THEN 2
    END as entityType
FROM
    Contacts as cnt
        INNER JOIN Objects as obj ON cnt.objectId = obj.id
        LEFT JOIN Companies as cmp ON cmp.id = obj.id
        LEFT JOIN Houses as hs ON cmp.id = hs.id

Also, note that entityId becomes objectId in this case.

Here is a CREATE-script for benchmarking and test:

CREATE TABLE IF NOT EXISTS `Objects` (
  `id` BIGINT NOT NULL AUTO_INCREMENT,
  `shared_property1` VARCHAR(45) NULL,
  `shared_property2` VARCHAR(45) NULL,
  `etc` VARCHAR(45) NULL,

  PRIMARY KEY (`id`))
ENGINE = InnoDB;

CREATE TABLE IF NOT EXISTS `Companies` (
  `id` BIGINT NOT NULL,
  `company_name` VARCHAR(300) NULL,
  `private_property1` VARCHAR(45) NULL,
  `private_property2` VARCHAR(45) NULL,
  `etc` VARCHAR(45) NULL,

  PRIMARY KEY (`id`),

  CONSTRAINT `FK_Companies_Objects`
    FOREIGN KEY (`id`)
    REFERENCES `Objects` (`id`)
    ON DELETE CASCADE
    ON UPDATE CASCADE)
ENGINE = InnoDB;

CREATE TABLE IF NOT EXISTS `Houses` (
  `id` BIGINT NOT NULL,
  `address` VARCHAR(300) NULL,
  `private_property1` VARCHAR(45) NULL,
  `private_property2` VARCHAR(45) NULL,
  `etc` VARCHAR(45) NULL,

  PRIMARY KEY (`id`),

  CONSTRAINT `FK_Houses_Objects`
    FOREIGN KEY (`id`)
    REFERENCES `Objects` (`id`)
    ON DELETE CASCADE
    ON UPDATE CASCADE)
ENGINE = InnoDB;

CREATE TABLE IF NOT EXISTS `Contacts` (
  `id` BIGINT NOT NULL AUTO_INCREMENT,
  `objectId` BIGINT NOT NULL,
  `name` VARCHAR(300) NULL,
  `phone` VARCHAR(300) NULL,
  `email` VARCHAR(300) NULL,

  PRIMARY KEY (`id`),

  INDEX `FK_Contacts_Objects` (`objectId` ASC),

  CONSTRAINT `FK_Contacts_Objects`
    FOREIGN KEY (`objectId`)
    REFERENCES `Objects` (`id`)
    ON DELETE CASCADE
    ON UPDATE CASCADE)
ENGINE = InnoDB;