Midrash su Salmi 89:21

It is written (Ruth 1, 2) Machlon and Chilyon, and in (I Chron. 4, 22) Joash and Saraph. Rab and Samuel differ. One said that their real names were Machlon and Chilyon. But why were they named Joash and Saraph? Joash, because they gave up hope of Divine redemption, and Saraph, because they were doomed to be burnt. And the other says their real names were Joash and Saraph. And why were they named Machlon and Chilyon? Machlon, because they made themselves very common by their emigration, and Chilyon, because they were doomed to destruction. We are taught in a Baraitha in accordance with the one who holds that Machlon and Chilyon were their real names. What is the meaning of the passage (Ib., ib.) And Joachim and the men of Goseba, and Joash and Saraph, who had dominion in Moab and Yoshbei lechem. And these are ancient things. Joachim, refers to Joshua, who had confirmed the oath which was given to the men of Gibeon; and the men of Goseba refers to Machlon and Chilyon. And why were they named Yoash and Saraph? Because they gave up hope of Divine remeption. Who had dominion in Moab, refers that they had married daughters of Moab. And Joshbei-lechem, refers to Ruth, who was a Mo'abite. And these are ancient things, means the Ancient of Days has decreed these things, as it is written (Ps. 99, 21) I have found David My servant. It is also written (Gen. 19, 15) And thy two daughters, that are found. These were the potters (hayozrim) and those that dwelt in plantations and sheepfolds. For the king's sake, to do his work, they dwelt there. Hayozrim, refers to the children of Jonadab b. Rechab, who preserved the oath of their father. (Ib. 4) In plantation, refers to King Solomon, who was a plant in his kingdom. Vegidroh (sheepfold), refers to the Sanhedrin, who had fenced the broken partition of Israel. For the king's sake, etc., refers to Ruth the Moabite, who lived to see the kingdom of Solomon, her great-grandson, as it is said (I Kings 2, 19) And placed a chair for the king's mother. Upon which R. Elazar said that it means to the mother of the kingdom.

Rabbi Abba bar Kahana began: “Tremble [rigzu] and do not sin…” (Psalms 4:5). David said before the Holy One, blessed be He: ‘How long will they be provoking [mitragzim] me, and saying: Does he not have flawed lineage? Does he not descend from Ruth the Moavite?’ ‘“…Say in your heart, on your bed…” (Psalms 4:5) – you also, do you not descend from two sisters?²⁵⁸You also have lineage that could be considered questionable, as Jacob married two sisters, Leah and Rachel. This would later be prohibited by the Torah. This is stated by David as a response to the elders of Judah, who had been taunting him due to his lineage. You, see what are your roots! “…and be silent, Sela” (Psalms 4:5). Tamar, too, whom Judah your grandfather took as a wife, is that not flawed lineage?²⁵⁹Judah married Tamar, who had previously been his daughter-in-law. This relationship too would later be prohibited by the Torah. And she was the daughter of Shem ben Noah.²⁶⁰The Sages identify him with Malkitzedek king of Shalem, who was a priest, and the Torah severely punishes the daughter of a priest who engages in licentiousness. Consequently, the lineage of her children Peretz and Zeraḥ were compromised by her act of having relations with Judah. What, do you have pedigree?’
Rabbi Yaakov bar Aviya said: Confront [argiz] your evil inclination and you will not sin. The Rabbis said: Anger your evil inclination and you will not sin.
“These are the generations of Peretz: Peretz begot Ḥetzron” (Ruth 4:18).
“These [eleh] are the generations of Peretz.” Rabbi Abba said: Anywhere that eleh is stated, it rejects what preceded it; ve’eleh, it adds to what preceded it. Bereshit Rabba 12 until “informed him that Ishmael repented,” end of quote>.
“Ḥetzron begot Ram and Ram begot Aminadav” (Ruth 4:19).
“Ḥetzron begot Ram.” But wasn’t Yeraḥme’el a previous son, [as it is stated:] “the sons of Ḥetzron who were born to him: Yeraḥme’el, Ram, and Keluvi” (I Chronicles 2:9)? Rather, he married a Canaanite woman in order to exalt himself [lehitater] through her, as it is written: “Yeraḥme’el had another wife, and her name was Atara” (I Chronicles 2:26).²⁶¹Since Yeraḥme’el married a Canaanite woman, the line of kingship did not come through him.
“Aminadav begot Naḥshon and Naḥshon begot Salma. Salmon begot Boaz and Boaz begot Oved” (Ruth 4:20–21).
“Ram begot Aminadav…and [Naḥshon begot…] Salmon.” To this point, they were the ladder for the princes;²⁶²Of the tribes. from this point on they were ladders to kings.²⁶³The name Salma, or Salmon, is similar to sulam, ladder. It appears in two different forms in these verses to allude to the change in status of the family: To this point, its members were tribal princes; from Boaz, Salmon’s son, the family was moving toward establishing the Israelite monarchy (Midrash HaMevoar). Rabbi Yitzḥak began: “Then [az] I said, behold, I have entered” (Psalms 40:8). I should have sung a song when I came.²⁶⁴When I was accepted into the congregation. Az means nothing other than song, as it is stated: “Then [az] Moses sang” (Exodus 15:1). I was included in “shall not enter,” and [yet] I entered.²⁶⁵David is cited as saying that he was assumed to be included within the prohibition against a Moavite convert entering the congregation, as he was a descendant of Ruth, but he was able to enter the congregation because of the acceptance of the halakha that this prohibition applies only to male converts.
“Oved begot Yishai and Yishai begot David” (Ruth 4:22).
“In the scroll of a book it is written about me” (Psalms 40:8). “In the scroll” – “whom You commanded that they should not enter into Your congregation” (Lamentations 1:10). “Of a book” – “An Ammonite and a Moavite shall not enter [into the assembly of the Lord]” (Deuteronomy 23:4). Not only did I enter, but in a scroll and a book it is written about me. In the scroll: Peretz, Ḥetzron, Ram, Aminadav, Naḥshon, Oved, Yishai, David.²⁶⁶The reference is to the verses in Ruth 4:18–22. In a book – “the Lord said: Arise, anoint him; for this is he” (I Samuel 16:12). Rabbi Huna says: It is written: “The Lord has provided me another [aḥer] offspring” (Genesis 4:25), an offspring that comes from another [aḥer] place, and who is this? It is the messianic king.²⁶⁷The messianic king is a descendant of David, and thus David is alluded to in the Torah itself.
Rabbi Berekhya and Rabbi Simon [said]: This is analogous to a king who would pass from place to place and a pearl fell from [the crown] on his head. The king and his entire entourage stopped there, and passersby would say: ‘What is the nature [of the stay] of the king and his entourage here?’ They saw and said: ‘A pearl fell from upon his head.’ What did he do? He piled up the dirt, and brought sifters, and sifted one pile but did not find it; the second, but did not find it; and the third, and he found it. They said: ‘The king found his pearl.’ So too, the Holy One blessed be He said to Abraham: “Go for yourself [lekha]” (Genesis 12:1) – It was you [lekha] that I was anticipating. What need was there to delineate the lineage: “Shem, Arpakhshad, Shelaḥ, Ever, Peleg… Naḥor, Teraḥ” (I Chronicles 1:24–26)? It was only for you. “Abram is [hu] Abraham” (I Chronicles 1:27)²⁶⁸It was he [hu] that God was anticipating, and for whom all the previous generations were listed. – “and You found his heart faithful before You” (Nehemiah 9:8). So too, the Holy One blessed be He said to David: ‘What need was there for Me to delineate the lineage: Peretz, Ḥetzron, Ram, Aminadav, Naḥshon, Oved, Yishai? Was it not for you?’ – “I have found David My servant” (Psalms 89:21).

Raba lectured: "What does the passage (Ps. 116, 16) Thou hadst loosened my fetters, mean? Thus said David before the Holy One, praised be He! — 'Sovereign of the Universe! Thou hast absolved two restrictions which were against me, they are Ruth the Moabite and Na'amah the Ammonite.'" Raba lectured: "What is the meaning of the passage (Ib. 40, 6) Many things hast Thou done, O Lord my God! Thy wonderful deeds and Thy thoughts toward us? It does not say toward me, but toward us. From this it may be inferred that Rehobo'am was sitting in David's lap, and the latter said that concerning you and me refer the two [above quoted] passages." Raba lectured: "What does the passage (Ib., ib., 8) Then said I: 'Lo, I come in the scroll of the book it is written down for me,' mean? David said: 'I thought that I came [to my royalty] just now by a mere incident, and I was not aware of the fact that, in the roll of the book [dating from Abraham,] it is written down concerning me; for it is written there (Gen. 19, 15) Hanimtza'oth (that are here), and here it is written (Ps. 89, 21) Matzathi (I have found), David My servant: with My holy oil," etc.